Patent Publication Number: US-11036439-B2

Title: Automated management of bundled applications

Description:
RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 16/167,049 filed Oct. 22, 2018, which is incorporated herein by reference for all purposes. 
     FIELD OF THE INVENTION 
     This invention relates to orchestration of roles in an application instantiated in a distributed storage and computation system. 
     BACKGROUND OF THE INVENTION 
     In many contexts, it is helpful to be able to return a database or distributed application to an original state or some intermediate state. In this manner, changes to the distributed application or other database configuration parameters may be tested without fear of corrupting critical data. 
     The systems and methods disclosed herein provide an improved approach for creating snapshots of a database and returning to a previous snapshot. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through use of the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of a network environment for implementing methods in accordance with an embodiment of the present invention; 
         FIG. 2  is a process flow diagram of a method for coordinating snapshot creation with compute nodes and storage nodes in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic diagram illustrating the storage of data within a storage node in accordance with an embodiment of the present invention; 
         FIG. 4  is a process flow diagram of a method for processing write requests in a storage node in accordance with an embodiment of the present invention; 
         FIG. 5  is a process flow diagram of a method for processing a snapshot instruction by a storage node in accordance with an embodiment of the present invention; 
         FIG. 6  is a process flow diagram of a method for performing garbage collection on segments in accordance with an embodiment of the present invention; 
         FIG. 7  is a process flow diagram of a method for reading data from a snapshot in accordance with an embodiment of the present invention; 
         FIG. 8  is a process flow diagram of a method for cloning a snapshot in accordance with an embodiment of the present invention; 
         FIG. 9  illustrates a snapshot hierarchy created in accordance with an embodiment of the present invention; 
         FIG. 10  is a process flow diagram of a method for rolling back to a prior snapshot in accordance with an embodiment of the present invention; 
         FIG. 11  illustrates the snapshot hierarchy of  FIG. 9  as modified according to the method of  FIG. 10  in accordance with an embodiment of the present invention; 
         FIG. 12  is a process flow diagram of a method for reading from a clone snapshot in accordance with an embodiment of the present invention; 
         FIG. 13  is a schematic block diagram of components for implementing orchestration of multi-role applications in accordance with an embodiment of the present invention; 
         FIG. 14  is a process flow diagram of a method for orchestrating the deployment of a multi-role application in accordance with an embodiment of the present invention; 
         FIG. 15  is a process flow diagram of a method for implementing provisioning constraints in accordance with an embodiment of the present invention; 
         FIG. 16  is a process flow diagram of a method for creating a snapshot of a multi-role application in accordance with an embodiment of the present invention; 
         FIG. 17  is a process flow diagram of a method for rolling back a multi-role application in accordance with an embodiment of the present invention; 
         FIG. 18  is a diagram illustrating the use of a layered file system to improve application portability in accordance with an embodiment of the present invention; 
         FIG. 19  is a process flow diagram of a method for creating and moving a portable application in accordance with an embodiment of the present invention; 
         FIG. 20  is a process flow diagram of a method for testing a distributed application in accordance with an embodiment of the present invention; 
         FIG. 21  is a schematic block diagram of components of a storage node in accordance with an embodiment of the present invention; 
         FIG. 22  is a process flow diagram of a method for assigning storage volumes to a disk of a storage node in accordance with an embodiment of the present invention; 
         FIG. 23  is a process flow diagram of a method for managing storage volumes of a bundled application in accordance with an embodiment of the present invention; 
         FIG. 24  is a schematic block diagram of data structures for managing a number of volumes per disk in accordance with an embodiment of the present invention; 
         FIG. 25  is a process flow diagram of a method for managing the number of volumes per disk in accordance with an embodiment of the present invention; and 
         FIG. 26  is a schematic block diagram of an example computing device suitable for implementing methods in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the methods disclosed herein may be performed using the illustrated network environment  100 . The network environment  100  includes a storage manager  102  that coordinates the creation of snapshots of storage volumes and maintains records of where snapshots are stored within the network environment  100 . In particular, the storage manager  102  may be connected by way of a network  104  to one or more storage nodes  106 , each storage node having one or more storage devices  108 , e.g. hard disk drives, flash memory, or other persistent or transitory memory. The network  104  may be a local area network (LAN), wide area network (WAN), or any other type of network including wired, fireless, fiber optic, or any other type of network connections. 
     One or more compute nodes  110  are also coupled to the network  104  and host user applications that generate read and write requests with respect to storage volumes managed by the storage manager  102  and stored within the memory devices  108  of the storage nodes  108 . 
     The methods disclosed herein ascribe certain functions to the storage manager  102 , storage nodes  106 , and compute node  110 . The methods disclosed herein are particularly useful for large scale deployment including large amounts of data distributed over many storage nodes  106  and accessed by many compute nodes  110 . However, the methods disclosed herein may also be implemented using a single computer implementing the functions ascribed herein to some or all of the storage manager  102 , storage nodes  106 , and compute node  110 . 
     Referring to  FIG. 2 , the illustrated method  200  may be performed in order to invoke the creation of a new snapshot. Other than a current snapshot, which is still subject to change, a snapshot captures the state of a storage volume at a moment in time and is not altered in response to subsequent writes to the storage volume. 
     The method  200  includes receiving, by the storage manager  102  a request to create a new snapshot for a storage volume. A storage volume as referred to herein may be a virtual storage volume that may divided into individual slices. For example, storage volumes as described herein may be 1 TB and be divided into 1 GB slices. In general, a slice and its snapshot are stored on a single storage node  106 , whereas a storage volume may have the slices thereof stored by multiple storage nodes  106 . 
     The request received at step  202  may be received from a human operator or generated automatically, such as according to backup scheduler executing on the storage manager  102  or some other computing device. The subsequent steps of the method  200  may be executed in response to receiving  202  the request 
     The method  200  may include transmitting  204  a quiesce instruction to all compute nodes  110  that are associated with the storage volume. For example, all compute nodes  110  that have pending write requests to the storage volume. In some embodiments, the storage manager  102  may store a mapping of compute nodes  110  to a particular storage volume used by the compute nodes  110 . Accordingly, step  204  may include sending  204  the quiesce instruction to all of these compute nodes. Alternatively, the instruction may be transmitted  204  to all compute nodes  110  and include an identifier of the storage volume. The compute nodes  110  may then suppress any write instructions referencing that storage volume. 
     The quiesce instruction instructs the compute nodes  110  that receive it to suppress  206  transmitting write requests to the storage nodes  106  for the storage volume referenced by the quiesce instruction. The quiesce instruction may further cause the compute nodes  110  that receive it to report  208  to the storage manager  102  when no write requests are pending for that storage volume, i.e. all write requests issued to one or more storage nodes  106  and referencing slices of that storage volume have been acknowledged by the one or more storage nodes  106 . 
     In response to receiving the report of step  208  from one or more compute nodes, e.g. all compute nodes that are mapped to the storage node that is the subject of the snapshot request of step  202 , the storage manager  102  transmits  210  an instruction to the storage nodes  106  associated with the storage volume to create a new snapshot of that storage volume. Step  210  may further include transmitting  210  an instruction to the compute nodes  110  associated with the storage volume to commence issuing write commands to the storage nodes  106  associated with the storage volume. In some embodiments, the instruction of step  110  may include an identifier of the new snapshot. Accordingly, subsequent input/output operations (IOPs) transmitted  214  from the compute nodes may reference that snapshot identifier. Likewise, the storage node  106  may associate the snapshot identifier with data subsequently written to the storage volume, as described in greater detail below. 
     In response to receiving  210  the instruction to create a new snapshot, each storage node  106  finalizes  212  segments associated with the current snapshot, which may include performing garbage collection, as described in greater detail below. In addition, subsequent IOPs received by the storage node may also be processed  216  using the new snapshot as the current snapshot, as is also described in greater detail below. 
     Referring to  FIG. 3 , the method by which slices are allocated, reassigned, written to, and read from may be understood with respect to the illustrated data storage scheme. The data of the storage scheme may be stored in transitory or persistent memory of the storage node  106 , such as in the storage devices  108 . 
     For each logical volume, the storage manager  102  may store and maintain a volume map  300 . For each slice in the logical volume, the volume map may include an entry including a node identifier  302  identifying the storage node  106  to which the slice is assigned and an offset  304  within the logical volume at which the slice begins. In some embodiments, slices are assigned both to a storage node  106  and a specific storage device hosted by the storage node  106 . Accordingly, the entry may further include a disk identifier of the storage node  106  referencing the specific storage device to which the slice is assigned. 
     The remaining data structures of  FIG. 3  are stored on each storage node  106 . The storage node  106  may store a slice map  308 . The slice map  308  may include entries including a local slice identifier  310  that uniquely identifies each slice of the storage node  106 , e.g. each slice of each storage device hosted by the storage node  106 . The entry may further include a volume identifier  312  that identifies the logical volume to which the local slice identifier  310  is assigned. The entry may further include the offset  304  within the logical volume of the slice of the logical volume assigned to the storage node  106 . 
     In some embodiments, an entry in the slice map  308  is created for a slice of the logical volume only after a write request is received that references the offset  304  for that slice. This further supports the implementation of overprovisioning such that slices may be assigned to a storage node  106  in excess of its actual capacity since the slice is only tied up in the slice map  308  when it is actually used. 
     The storage node  106  may further store and maintain a segment map  314 . The segment map  314  includes entries either including or corresponding to a particular physical segment identifier (PSID)  316 . For example, the segment map  314  may be in an area of memory such that each address in that area corresponds to one PSID  316  such that the entry does not actually need to include the PSID  316 . The entries of the segment map  314  may further include a slice identifier  310  that identifies a local slice of the storage node  106  to which the PSID  316  has been assigned. The entry may further include a virtual segment identifier (VSID)  318 . As described in greater detail below, each time a segment is assigned to logical volume and a slice of a logical volume, it may be assigned a VSID  318  such that the VSIDs  318  increase in value monotonically in order of assignment. In this manner, the most recent PSID  316  assigned to a logical volume and slice of a logical volume may easily be determined by the magnitude of the VSIDs  318  mapped to the PSIDs  316 . In some embodiments, VSIDs  318  are assigned in a monotonically increasing series for all segments assigned to volume ID  312 . In other embodiments, each offset  304  and its corresponding slice ID  310  is assigned VSIDs separately, such that each slice ID  310  has its own corresponding series of monotonically increasing VSIDs  318  assigned to segments allocated to that slice ID  310 . 
     The entries of the segment map  314  may further include a data offset  320  for the PSID  316  of that entry. As described in greater detail below, when data is written to a segment it may be written at a first open position from a first end of the segment. Accordingly, the data offset  320  may indicate the location of this first open position in the segment. The data offset  320  for a segment may therefore be updated each time data is written to the segment to indicate where the new first open position is. 
     The entries of the segment map  314  may further include a metadata offset  322 . As described in detail below, for each write request written to a segment, a metadata entry may be stored in that segment at a first open position from a second end of the segment opposite the first end. Accordingly, the metadata offset  322  in an entry of the segment map  314  may indicate a location of this first open position of the segment corresponding to the entry. 
     Each PSID  316  corresponds to a physical segment  324  on a device hosted by the storage node  106 . As shown, data payloads  326  from various write requests are written to the physical segment  324  starting from a first end (left) of the physical segment. The physical segment may further store index pages  328  such that index pages are written starting from a second end (right) of the physical segment  324 . 
     Each index page  328  may include a header  330 . The header  330  may be coded data that enables identification of a start of an index page  328 . The entries of the index page  328  each correspond to one of the data payloads  326  and are written in the same order as the data payloads  326 . Each entry may include a logical block address (LBA)  332 . The LBA  332  indicates an offset within the logical volume to which the data payload corresponds. The LBA  332  may indicate an offset within a slice of the logical volume. For example, inasmuch as the PSID  316  is mapped to a slice ID  310  that is mapped to an offset  304  within a particular volume ID  312 , maps  308  and  314 , and an LBA  332  within the slice may be mapped to the corresponding offset  304  to obtain a fully resolved address within the logical volume. 
     In some embodiments, the entries of the index page  328  may further include a physical offset  334  of the data payload  326  corresponding to that entry. Alternatively or additionally, the entries of the index page  328  may include a size  336  of the data payload  326  corresponding to the entry. In this manner, the offset to the start of a data payload  326  for an entry may be obtained by adding up the sizes  336  of previously written entries in the index pages  328 . 
     The metadata offset  322  may point to the last index page  328  (furthest from right in illustrated example) and may further point to the first open entry in the last index page  328 . In this manner, for each write request, the metadata entry for that request may be written to the first open position in the last index page  328 . If all of the index pages  328  are full, a new index page  328  may be created and stored at the first open position from the second end and the metadata for the write request may be added at the first open position in that index page  328 . 
     The storage node  106  may further store and maintain a block map  338 . A block map  338  may be maintained for each logical volume and/or for each slice offset of each logical volume, e.g. for each local slice ID  310  which is mapped to a slice offset and logical volume by slice map  308 . The entries of the block map  338  map include entries corresponding to each LBA  332  within the logical volume or slice of the logical volume. The entries may include the LBA  332  itself or may be stored at a location within the block map corresponding to an LBA  332 . 
     The entry for each LBA  332  may include the PSID  316  identifying the physical segment  324  to which a write request referencing that LBA was last written. In some embodiments, the entry for each LBA  332  may further indicate the physical offset  334  within that physical segment  324  to which the data for that LBA was written. Alternatively, the physical offset  324  may be obtained from the index pages  328  of that physical segment. As data is written to an LBA  332 , the entry for that LBA  332  may be overwritten to indicate the physical segment  324  and physical offset  334  within that segment  324  to which the most recent data was written. 
     In embodiments implementing multiple snapshots for a volume and slice of a volume, the segment map  314  may additionally include a snapshot ID  340  identifying the snapshot to which the PSID  316  has been assigned. In particular, each time a segment is allocated to a volume and slice of a volume, the current snapshot identifier for that volume and slice of a volume will be included as the snapshot ID  340  for that PSID  316 . 
     In response to an instruction to create a new snapshot for a volume and slice of a volume, the storage node  106  will store the new current snapshot identifier, e.g. increment the previously stored current snapshot ID  340 , and subsequently allocated segments will include the current snapshot ID  340 . PSIDs  316  that are not filled and are allocated to the previous snapshot ID  340  may no longer be written to. Instead, they may be finalized or subject to garbage collection (see  FIGS. 5 and 6 ). 
       FIG. 4  illustrates a method  400  for executing write instructions by a storage node  106 , such as write instructions received from an application executing on a compute node  110 . 
     The method  400  includes receiving  402  a write request. The write request may include payload data, payload data size, and an LBA as well as fields such as a slice identifier, a volume identifier, and a snapshot identifier. Where a slice identifier is included, the LBA may be an offset within the slice, otherwise the LBA may be an address within the storage volume. 
     The method  400  may include evaluating  404  whether a PSID  316  is allocated to the snapshot referenced in the write request and whether the physical segment  324  corresponding to the PSID  316  (“the current segment”) has space for the payload data. In some embodiments, as write requests are performed with respect to a PSID  316 , the amount of data written as data  326  and index pages  328  may be tracked, such as by way of the data offset  320  and metadata offset  322  pointers. Accordingly, if the amount of previously-written data  326  and the number of allocated index pages  328  plus the size of the payload data and its corresponding metadata entry exceeds the capacity of the current segment it may be determined to be full at step  404 . 
     If the current segment is determined  404  to be full, the method  400  may include allocating  406  a new PSID  316  as the current PSID  316  and its corresponding physical segment  324  as the current segment for the snapshot referenced in the write request. In some embodiments, the status of PSIDs  316  of the physical storage devices  108  may be flagged in the segment map  314  as allocated or free as a result of allocation and garbage collection, which is discussed below. Accordingly, a free PSID  316  may be identified in the segment map  314  and flagged as allocated. 
     The segment map  314  may also be updated  408  to include a slice ID  310  and snapshot ID  340  mapping the current PSID  316  to the snapshot ID, volume ID  312 , and offset  304  included in the write request. Upon allocation, the current PSID  316  may also be mapped to a VSID (virtual segment identifier)  318  that will be a number higher than previously VSIDs  318  such that the VSIDs increase monotonically, subject, of course, to the size limit of the field used to store the VSID  318 . However, the size of the field may be sufficiently large that it is not limiting in most situations. 
     The method  400  may include writing  410  the payload data to the current segment. As described above, this may include writing  410  payload data  326  to the free location closest to the first end of the current segment. 
     The method  400  may further include writing  412  a metadata entry to the current segment. This may include writing the metadata entry (LBA, size) to the first free location closest to the second end of the current segment. Alternatively, this may include writing the metadata entry to the first free location in an index page  328  that has room for it or creating a new index page  328  located adjacent a previous index page  328 . Steps  410 ,  412  may include updating one or more pointers or table that indicates an amount of space available in the physical segment, such as a pointer  320  to the first free address closest to the first end and a pointer  322  to the first free address closest to the second end, which may be the first free address before the last index page  328  and/or the first free address in the last index page. In particular, these pointers may be maintained as the data offset  320  and metadata offset in the segment map  314  for the current PSID  316 . 
     The method  400  may further include updating  416  the block map  338  for the current snapshot. In particular, for each LBA  332  referenced in the write request, an entry in the block map  338  for that LBA  332  may be updated to reference the current PSID  316 . A write request may write to a range of LBAs  332 . Accordingly, the entry for each LBA  332  in that range may be updated to refer to the current PSID  316 . 
     Updating the block map  338  may include evaluating  414  whether an entry for a given LBA  332  referenced in the write request already exists in the block map  338 . If so, then that entry is overwritten  418  to refer to the current PSID  316 . If not, an entry is updated  416  in the block map  318  that maps the LBA  332  to the current PSID  316 . In this manner, the block map  338  only references LBAs  332  that are actually written to, which may be less than all of the LBAs  332  of a storage volume or slice. In other embodiments, the block map  338  is of fixed size and includes and entry for each LBA  332  regardless of whether it has been written to previously. The block map  338  may also be updated to include the physical offset  334  within the current segment to which the data  326  from the write request was written. 
     In some embodiments, the storage node  106  may execute multiple write requests in parallel for the same LBA  332 . Accordingly, it is possible that a later write can complete first and update the block map  338  whereas a previous write request to the same LBA  332  completes later. The data of the previous write request is therefore stale and the block map  338  should not be updated. 
     Suppressing of updating the block map  338  may be achieved by using the VSIDs  318  and physical offset  334 . When executing a write request for an LBA, the VSID  318  mapped to the segment  324  and the physical offset  334  to which the data is to be, or was, written may be compared to the VSID  318  and offset  334  corresponding to the entry in the block map  338  for the LBA  332 . If the VSID  318  mapped in the segment map  314  to the PSID  316  in the entry of the block map  338  corresponding to the LBA  332 , then the block map  338  will not be updated. Likewise, if the VSID  318  corresponding to the PSID  316  in the block map  338  is the same as the VSID  318  for the write request and the physical offset  334  in the block map  338  is higher than the offset  334  to which the data of the write request is to be or was written, the block map  338  will not be updated for the write request. 
     As a result of steps  414 - 418 , the block map  338  only lists the PSID  316  where the valid data for a given LBA  332  is stored. Accordingly, only the index pages  328  of the physical segment  324  mapped to the PSID  316  listed in the block map  338  need be searched to find the data for a given LBA  332 . In instances where the physical offset  334  is stored in the block map  338 , no searching is required. 
       FIG. 5  illustrates a method  500  executed by a storage node  106  in response to the new snapshot instruction of step  210  for a storage volume. The method  500  may be executed in response to an explicit instruction to create a new snapshot or in response to a write request that includes a new snapshot ID  340 . The method  500  may also be executed with respect to a current snapshot that is still being addressed by new write requests. For example, the method  500  may be executed periodically or be triggered based on usage. 
     The method  500  may include allocating  502  a new PSID  316  and its corresponding physical segment  324  as the current PSID  316  and current segment for the storage volume, e.g., by including a slice ID  310  corresponding to a volume ID  312  and offset  304  included in the new snapshot instruction or the write request referencing the new snapshot ID  340 . Allocating  502  a new segment may include updating  504  an entry in the segment map  314  that maps the current PSID  316  to the snapshot ID  340  and a slice ID  310  corresponding to a volume ID  312  and offset  304  included in the new snapshot instruction. 
     As noted above, when a PSID  316  is allocated, the VSID  318  for that PSID  316  may be a number higher than all VSIDs  318  previously assigned to that volume ID  312 , and possibly to that slice ID  310  (where slices have separate series of VSIDs  318 ). The snapshot ID  340  of the new snapshot may be included in the new snapshot instruction or the storage node  106  may simply assign a new snapshot ID that is the previous snapshot ID  340  plus one. 
     The method  500  may further include finalizing  506  and performing garbage collection with respect to PSIDs  316  mapped to one or more previous snapshots IDs  340  for the volume ID  312  in the segment map  314 , e.g., PSIDs  316  assigned to the snapshot ID  340  that was the current snapshot immediately before the new snapshot instruction was received. 
       FIG. 6  illustrates a method  600  for finalizing and performing garbage collection with respect to segment IDs  340  for a snapshot (“the subject snapshot”), which may include the current snapshot or a previous snapshot. The method  600  may include marking  602  as valid latest-written data for an LBA  332  in the PSID  316  having the highest VSID  318  in the segment map  314  and to which data was written for that LBA  332 . Marking  602  data as valid may include making an entry in a separate table that lists the location of valid data or entries for metadata in a given physical segment  324  or setting a flag in the metadata entries stored in the index pages  328  of a physical segment  324 , e.g., a flag that indicates that the data referenced by that metadata is invalid or valid. 
     Note that the block map  338  records the PSID  316  for the latest version of the data written to a given LBA  332 . Accordingly, any references to that LBA  332  in the physical segment  324  of a PSID  316  mapped to a lower-numbered VSID  318  may be marked  604  as invalid. For the physical segment  324  of the PSID  316  in the block map  338  for a given LBA  332 , the last metadata entry for that LBA  332  may be found and marked as valid, i.e. the last entry referencing the LBA  332  in the index page  328  that is the last index page  328  including a reference to the LBA  332 . Any other references to the LBA  332  in the physical segment  324  may be marked  604  as invalid. Note that the physical offset  334  for the LBA  332  may be included in the block map  334 , so all metadata entries not corresponding to that physical offset  334  may be marked as invalid. 
     The method  600  may then include processing  606  each segment ID S of the PSIDs  316  mapped to the subject snapshot according to steps  608 - 620 . In some embodiments, the processing of step  606  may exclude a current PSID  316 , i.e. the last PSID  302  assigned to the subject snapshot. As described below, garbage collection may include writing valid data from a segment to a new segment. Accordingly, step  606  may commence with the PSID  316  having the lowest-valued VSID  318  for the subject snapshot. As any segments  324  are filled according to the garbage collection process, they may also be evaluated to be finalized or subject to garbage collection as described below. 
     The method  600  may include evaluating  608  whether garbage collection is needed for the segment ID S. This may include comparing the amount of valid data in the physical segment  324  for the segment ID S to a threshold. For example, if only 40% of the data stored in the physical segment  324  for the segment ID S has been marked valid, then garbage collection may be determined to be necessary. Other thresholds may be used, such as value between 30% and 80%. In other embodiments, the amount of valid data is compared to the size of the physical segment  324 , e.g., the segment ID S is determined to need garbage collection if the amount of valid data is less than X % of the size of the physical segment  324 , where X is a value between 30 and 80, such as 40. 
     If garbage collection is determined  608  not to be needed, the method  600  may include finalizing  610  the segment ID S. Finalizing may include flagging the segment ID S in the segment map  314  as full and no longer available to be written to. This flag may be stored in another table that lists finalized PSIDs  316 . 
     If garbage collection is determined  608  to be needed, then the method  600  may include writing  612  the valid data to a new segment. For example, if the valid data may be written to a current PSID  316 , i.e. the most-recently allocated PSID  316  for the subject snapshot, until its corresponding physical segment  324  full. If there is no room in the physical segment  324  for the current PSID  316 , step  612  may include assigning a new PSID  316  as the current PSID  316  for the subject snapshot. The valid data, or remaining valid data, may then be written to the physical segment  324  corresponding to the current PSID  316  for the subject snapshot. 
     Note that writing  612  the valid data to the new segment maybe processed in the same manner as for any other write request (see  FIG. 4 ) except that the snapshot ID used will be the snapshot ID  340  of the subject snapshot, which may not be the current snapshot ID. In particular, the manner in which the new PSID  316  is allocated to the subject snapshot may be performed in the same manner described above with respect to steps  406 - 48  of  FIG. 4 . Likewise, the manner in which the valid data is written to the current segment may be performed in the same manner as for steps  410 - 412  of  FIG. 4 . In some embodiments, writing of valid data to a new segment as part of garbage collection may also include updating the block map with the new location of the data for an LBA  332 , such as according to steps  414 - 418  of  FIG. 4 . When the physical segment  324  of the current PSID  316  is found to be full, it may itself be subject to the process  600  by which it is finalized or subject to garbage collection. 
     After the valid data is written to a new segment, the method  600  may further include freeing  614  the PSID Sin the segment map  314 , e.g., marking the entry in segment map  314  corresponding to PSID S as free. 
     The process of garbage collection may be simplified for PSIDs  316  that are associated with the subject snapshot in the segment map  314  but are not listed in the block map  338  with respect to any LBA  332 . The physical segments  324  of such PSIDs  316  do not store any valid data. Entries for such PSIDs  316  in the segment map  314  may therefore simply be deleted and marked as free in the segment map  314   
       FIG. 7  illustrates a method  700  that may be executed by a storage node  106  in response to a read request. The read request may be received from an application executing on a compute node  110 . The read request may include such information as a snapshot ID, volume ID (and/or slice ID), LBA, and size (e.g. number of 4 KB blocks to read). 
     The following steps of the method  700  may be initially executed using the snapshot ID  340  included in the read request as “the subject snapshot,” i.e., the snapshot that is currently being processed to search for requested data. The method  700  includes receiving  702  the read request by the storage node  106  and identifying  704  one or more PSIDs  316  in the segment map  314  assigned to the subject snapshot and searching  706  the metadata entries for these PSIDs  316  for references to the LBA  332  included in the read request. 
     The searching of step  706  may be performed in order of decreasing VSID  318 , i.e. such that the metadata entries for the last allocated PSID  316  is searched first. In this manner, if reference to the LBA  332  is found, the metadata of any previously-allocated PSIDs  316  does not need to be searched. 
     Searching  706  the metadata for a PSID  316  may include searching one or more index pages  328  of the physical segment  324  corresponding to the PSID  316 . As noted above, one or more index pages  328  are stored at the second end of the physical segment  324  and entries are added to the index pages  328  in the order they are received. Accordingly, the last-written metadata including the LBA  332  in the last index page  328  (furthest from the second end of the physical segment  324 ) in which the LBA  332  is found will correspond to the valid data for that LBA  332 . To locate the data  326  corresponding to the last-written metadata for the LBA  332  in the physical segment  324 , the sizes  336  for all previously-written metadata entries may be summed to find a start address in the physical segment  324  for the data  326 . Alternatively, if the physical offset  334  is included, then the data  326  corresponding to the metadata may be located without summing the sizes  336 . 
     If reference to the LBA  332  is found  708  in the physical segment  324  for any of the PSIDs  316  allocated to the subject snapshot, the data  326  corresponding to the last-written metadata entry including that LBA  332  in the physical segment  324  mapped to the PSID  316  having the highest VSID  318  of all PSIDs  316  in which the LBA is found will be returned  710  to the application that issued the read request. 
     If the LBA  332  is not found in the metadata entries for any of the PSIDs  316  mapped to subject snapshot, the method  700  may include evaluating  712  whether the subject snapshot is the earliest snapshot for the storage volume of the read request on the storage node  106 . If so, then the data requested is not available to be read and the method  700  may include returning  714  a “data not found” message or otherwise indicating to the requesting application that the data is not available. 
     If an earlier snapshot than the subject snapshot is present for the storage volume on the storage node  106 , e.g., there exists at least one PSID  316  mapped to a snapshot ID  340  that is lower than the snapshot ID  340  of the subject snapshot ID, then the immediately preceding snapshot ID  340  will be set  716  to be the subject snapshot and processing will continue at step  704 , i.e. the PSIDs  316  mapped to the subject snapshot will be searched for the LBA  332  in the read request as described above. 
     The method  700  is particularly suited for reading data from snapshots other than the current snapshot that is currently being written to. In the case of a read request from the current snapshot, the block map  338  may map each LBA  332  to the PSID  316  in which the valid data for that LBA  332  is written. Accordingly, for such embodiments, step  704  may include retrieving the PSID  332  for the LBA  332  in the write request from the block map  338  and only searching  706  the metadata corresponding to that PSID  316 . Where the block map  338  stores a physical offset  334 , then the data is retrieved from that physical offset within the physical segment  314  of the PSID  336  mapped to the LBA  332  of the read request. 
     In some embodiments, the block map  332  may be generated for a snapshot other than the current snapshot in order to facilitate executing read requests, such as where a large number of read requests are anticipated in order to reduce latency. This may include searching the index pages  328  of the segments  324  allocated to the subject snapshot and its preceding snapshots to identify, for each LBA  332  to which data has been written, the PSID  316  having the highest VSID  318  of the PSIDs  316  having physical segments  324  storing data written to the each LBA  332 . This PSID  316  may then be written to the block map  318  for the each LBA  332 . Likewise, the physical offset  334  of the last-written data for that LBA  332  within the physical segment  324  for that PSID  316  may be identified as described above (e.g., as described above with respect to steps  704 - 716 ). 
     Referring to  FIG. 8 , in some instances it may be beneficial to clone a storage volume. This may include capturing a current state of a principal copy of a storage volume and making changes to it without affecting the principal copy of the storage volume. For purposes of this disclosure a “principal copy” or “principal snapshot” of a storage volume refers to an actual production copy that is part of a series of snapshots that is considered by the user to be the current, official, or most up-to-date copy of the storage volume. In contrast, a clone snapshot is a snapshot created for experimentation or evaluation but changes to it are not intended by the user to become part of the production copy of the storage volume. Stated differently, only one snapshot may be a principal snapshot with respect to an immediately preceding snapshot, independent of the purpose of the snapshot. Any other snapshots that are immediate descendants of the immediately preceding snapshot are clone snapshots. 
     The illustrated method  800  may be executed by the storage manager  102  and one or more storage nodes  106  in order to implement this functionality. The method  800  may include receiving  802  a clone instruction and executing the remaining steps of the method  800  in response to the clone instruction. The clone instruction may be received by the storage manager  102  from a user or be generated according to a script or other program executing on the storage manager  102  or a remote computing device in communication with the storage manager  102 . 
     The method  800  may include recording  804  a clone branch in a snapshot tree. For example, referring to  FIG. 9 , in some embodiments, for each snapshot that is created for a storage volume, the storage manager  102  may create a node S 1 -S 5  in a snapshot hierarchy  900 . In response to a clone instruction, the storage manager  102  may create a clone snapshot and branch to a node A 1  representing the clone snapshot. In the illustrated example, a clone instruction was received with respect to the snapshot of node S 2 . This resulted in the creation of clone snapshot represented by node A 1  that branches from node S 2 . Note node S 3  and its descendants are also connected to node S 2  in the hierarchy. 
     In some embodiments, the clone instruction may specify which snapshot the clone snapshot is of In other embodiments, the clone instruction may be inferred to be a snapshot of a current snapshot. In such embodiments, a new principal snapshot may be created and become the current snapshot. The previous snapshot will then be finalized and be subject to garbage collection as described above. The clone will then branch from the previous snapshot. In the illustrated example, if node S 2  represented the current snapshot, then a new snapshot represented by node S 3  would be created. The snapshot of node S 2  would then be finalized and subject to garbage collection and clone snapshot represented by A 1  would be created and node A 1  would be added to the hierarchy as a descendent of node S 2 . 
     In some embodiments, the clone node A 1 , and possibly its descendants A 2  to A 4  (representing subsequent snapshots of the clone snapshot), may be distinguished from the nodes S 1  to S 5  representing principal snapshots, such as by means of a flag, a classification of the connection between the node A 1  and node S 2  that is its immediate ancestor, or by storing data defining node A 1  in a separate data structure. 
     Following creation of a clone snapshot, other principal snapshots of the storage volume may be created and added to represented in the hierarchy by one or more nodes S 2  to S 5 . A clone may be created of any of these snapshots and represented by additional clone nodes. In the illustrated example, node B 1  represents a clone snapshot of the snapshot represented by node S 4 . Subsequent snapshots of the clone snapshot are represented by nodes B 1  to B 3 . 
     Referring again to  FIG. 8 , the creation of a clone snapshot on the storage node  106  may be performed in the identical manner as for any other snapshot, such as according to the methods of  FIGS. 2 through 6 . In particular, one or more segments  806  may be allocated to the clone snapshot on storage nodes  106  storing slices of the cloned storage volume and mapped to the clone snapshot. IOPs referencing the clone snapshot may be executed  808 , such as according to the method  400  of  FIG. 4 . 
     In some instances, it may be desirable to store a clone snapshot on a different storage node  106  than the principal snapshots. Accordingly, the method  800  may include allocating  806  segments to the clone snapshot on the different storage node  106 . This may be invoked by sending a new snapshot instruction referencing the clone snapshot (i.e., an identifier of the clone snapshot) to the different storage node  106  and instructing one or more compute nodes  110  to route IOPs for the clone snapshot to the different storage node  106 . 
     The storage node  102  may store in each node of the hierarchy, data identifying one or more storage nodes  106  that store data for the snapshot represented by that node of the hierarchy. For example, each node may store or have associated therewith one or more identifiers of storage nodes  106  that store a particular snapshot ID for a particular volume ID. The node may further map one or more slice IDs (e.g., slice offsets) of a storage volume to one storage nodes  106  storing data for that slice ID and the snapshots for that slice ID. 
     Referring to  FIG. 10 , one of the benefits of snapshots is the ability to capture the state of a storage volume such that it can be restored at a later time.  FIG. 10  illustrates a method  1000  for rolling back a storage volume to a previous snapshot, particularly for a storage volume having one or more clone snapshots. 
     The method  1000  includes receiving  1002 , by the storage manager  102 , an instruction to rollback a storage volume to a particular snapshot SN. The method  1000  may then include processing  1004  each snapshot that is a represented by a descendent node of the node representing snapshot SN in the snapshot hierarchy, i.e. snapshots SN+1 to SMAX, where SMAX is the last principal snapshot that is a descendent of snapshot SN (each “descendent snapshot”). For each descendent snapshot, processing  1004  may include evaluating  1006  whether the each descendent is an ancestor of a node representing a clone snapshot. If not, then the storage manager  102  may instruct all storage nodes  106  storing segments mapped to the descendent snapshot to free  1008  these segments, i.e. delete entries from the segment map referencing the descendent snapshot and marking corresponding PSIDs  316  as free in the segment map  314 . 
     If the descendent snapshot is found  1006  to be an ancestor of a clone snapshot, then step  1008  is not performed and the snapshot and any segments allocated to it are retained. 
       FIG. 11  illustrates the snapshot hierarchy following execution of the method  1000  with respect to the snapshot represented by node S 3 . As is apparent, snapshot S 5  has been removed from the hierarchy and any segments corresponding to these snapshots will have been freed on one or more storage nodes  106 . 
     However, since node S 4  is an ancestor of clone node B 1 , it is not removed and segments corresponding to it are not freed on one or more storage nodes in response to the roll back instruction. Inasmuch as each snapshot contains only data written to the storage volume after it was created, previous snapshots may be required to recreate the storage volume. Accordingly, the snapshots of nodes S 3  to S 1  are needed to create the snapshot of the storage volume corresponding to node B 1 . 
     Subsequent principal snapshots of the storage volume will be added as descendants of the node to which the storage volume was rolled back. In the illustrated example, a new principal snapshot is represented by node S 6  that is an immediate descendent of node S 3 . Node S 4  is only present due to clone node B 1  and therefore may itself be classified as a clone node in the hierarchy in response to the rollback instruction of step  1002 . 
     Note that  FIG. 11  is a simple representation of a hierarchy. There could be any number of clone snapshots, clones of clone snapshots and descendent snapshots of any of these snapshots represented by nodes of a hierarchy. Accordingly, to roll back to a particular snapshot of a clone, the method  1000  is the same, except that descendants of the clone snapshot are treated the same as principal snapshots and clones of any of these descendants are treated the same as a clone snapshot. 
     Referring to  FIG. 12 , the illustrated method  1200  may be used to execute a read request with respect to a storage volume that is represented by a hierarchy generated as described above with respect to  FIGS. 8 through 11 . The illustrated method  1200  may also be executed with respect to a storage volume that includes only principal snapshots that are distributed across multiple storage nodes, i.e., all the segments corresponding to snapshots of the same slice of the storage volume are not located on the same storage node  106 . In that case, the hierarchy stored on the storage manager  102  stores the location of the segments for each snapshot and therefore enables them to be located. 
     The method  1200  may be executed by a storage node  106  (“the current storage node”) with information retrieved from the storage manager  102  as noted below. The method  1200  may include receiving  1202  a read request, which may include such information as a snapshot ID, volume ID (and/or slice ID), LBA, and size (e.g. number of 4 KB blocks to read). 
     Note that the read request may be issued by an application executing on a compute node  110 . The compute node  110  may determine which storage node  106  to transmit the read request using information from the storage manager  102 . For example, the compute node  110  may transmit a request to obtain an identifier for the storage node  102  storing data for a particular slice and snapshot of a storage volume. The storage manager may then obtain an identifier and/or address for the storage node  106  storing that snapshot and slice of the storage volume from the hierarchical representation of the storage volume and return it to the requesting compute node  110 . For example, the storage manager  102  may retrieve this information from the node in the hierarchy representing the snapshot included in the read request. 
     In response to the read request, the current storage node performs the algorithm illustrated by subsequent steps of the method  1200 . In particular, the method  1200  may include identifying  1204  segments assigned to the snapshot ID of the read request in the segment (“the subject snapshot”). 
     The method  1200  may include searching  1206  the metadata of the segments identified in step  1204  for the LBA of the read request. If the LBA is found, the data from the highest numbered segment having the LBA in its metadata is returned, i.e. the data that corresponds to the last-written metadata entry including the LBA. 
     If the LBA is not found in any of the segments mapped to subject snapshot, then the method  1200  may include evaluating  1212  whether the subject snapshot is the earliest snapshot on the current storage node. If not, then steps processing continues at step  1204  with the previous snapshot set  1214  as the subject snapshot. 
     Steps  1204 - 1214  may be performed in the same manner as for steps  704 - 714  of the method  700 , including the various modifications and variations described above with respect to the method  700 . 
     In contrast to the method  700 , if the LBA is not found in any of the segments corresponding to the subject snapshot for any of the snapshots evaluated, then the method  1200  may include requesting  1216  a location, e.g. storage node identifier, where an earlier snapshot for the volume ID or slice ID is stored. In response to this request, the storage manager  102  determines an identifier of a storage node  106  storing the snapshot corresponding to the immediate ancestor of the earliest snapshot stored on the current storage node in the hierarchy. The storage manager  102  may determine an identifier of the storage node  106  relating to the immediate-ancestor snapshot and that stores data for a slice ID and volume ID of the read request as recorded for the ancestor nearest ancestor node in the hierarchy of the node corresponding to the earliest snapshot stored on the current storage node. 
     If the current storage node is found  1218  to be the earliest snapshot for the storage volume ID and/or slice ID of the read request, then the data the storage manager  102  may report this fact to the storage node, which will then return  1220  a message indicating that the requested LBA is not available for reading, such as in the same manner as step  714  of the method  700 . 
     If another storage node stores an earlier snapshot for the volume ID and/or slice ID of the read request, then the read request may be transmitted  1222  to this next storage node by either the current storage node or the storage manager  102 . The processing may then continue at step  1202  with the next storage node as the current storage node. The read request transmitted at step  1222  may have a snapshot ID set to the latest snapshot ID for the storage volume ID and or slice ID of the original read request. 
     The method  1200  may be performed repeatedly across multiple storage nodes  106  until the earliest snapshot is encountered or the LBA of the read request is located. 
     Referring to  FIG. 13 , storage according to the above-described methods and systems may be incorporated into an application-orchestration approach. In the illustrates approach, an orchestration layer  1300  implements a bundled application  1302  including a plurality of roles. In the following description, “bundled application” refers to a bundle of applications as implemented using the orchestration layer. A “role” is an instance of an executable that is managed by the orchestration layer as described herein as part of the bundled application. Accordingly, a “role” may itself be a standalone application, such as a database, webserver, blogging application, or any other application. Examples of roles include the roles used to implement multi-role applications such as CASSANDRA, HADOOP, SPARK, DRUID, SQL database, ORACLE database, MONGODB database, WORDPRESS, and the like. For example, in HADOOP, roles may include one or more of a name node, data node, zookeeper, and AMBARI server. 
     The orchestration layer  1300  may implement a bundled application  1302  defining roles and relationships between roles as described in greater detail below. The orchestration layer  1300  may execute on a computing device of a distributed computing system (see e.g.,  FIG. 1 ), such as on a compute node  110 , storage node  106 , a computing device executing the functions of the storage manager  102 , or some other computing device. Accordingly, actions performed by the orchestration layer  1300  may be interpreted as being performed by the computing device executing the orchestration layer  1300 . 
     The bundled application  1302  may include a manifest  1304  that defines the roles of the bundled application  1302 , which may include identifiers of roles and possibly a number of instances for each role identified. The manifest  1304  may define dynamic functions define how the number of instances of particular role may grow or shrink depending on usage. The orchestration layer  1300  may then create or remove instances for a role as described below as indicated by usage and one or more functions for that role. The manifest  1304  may define a topology of the bundled application  1302 , i.e. the relationship between roles, such as services of a role that are accessed by another role. 
     The bundled application  1302  may include provisioning  1306 . The provisioning  1306  defines the resources of storage nodes  106  and compute nodes  110  required to implement the bundle. The provisioning  1306  may define resources for the bundle as a whole or for individual roles. Resources may include a number of processors (e.g., processing cores), an amount of memory (e.g., RAM (random access memory), an amount of storage (e.g., GB (gigabytes) on a HDD (Hard Disk Drive) or SSD (Solid State Drive)). As described below, these resources may be provisioned in a virtualized manner such that the bundled application  1302  and individual roles  1312  are not informed of the actual location or processing and storage resources and are relieved from any responsibility for managing such resources. In particular, storage resources may be virtualized by the storage manager  102  using the methods described above such that storage volumes are allocated and used without requiring the bundled application  1302  or roles to manage the underlying storage nodes  106  and storage device  108  on which the data of the storage volumes is written. 
     Provisioning  1306  may include static specification of resources and may also include dynamic provisioning functions that will invoke allocation of resources in response to usage of the bundled application. For example, as a database fills up, additional storage volumes may be allocated. As usage of a bundled application increases, additional processing cores and memory may be allocated to reduce latency. 
     A bundled application  1302  may further include configuration parameters  1308 . Configuration parameters may include variables and settings for each role of the bundle. The configuration parameters are defined by the developer of the role and therefore may include any example of such parameters for any application known in the art. The configuration parameters may be dynamic or static. For example, some parameters may be dependent on resources such as an amount of memory, processing cores, or storage. Accordingly, these parameters may be defined as a function of these resources. The orchestration layer will then update such parameters according to the function in response to changes in provisioning of those resources that are inputs to the function. For example, CASSANDRA defines a variable Max_Heap_Size that is normally set to half the memory limit. Accordingly, as the memory provisioned for a CASSANDRA role increases, the value of Max_Heap_Size may be increased to half the increased memory. 
     The bundled application  1302  may further include action hooks  1310  for various actions that may be taken with respect to the bundled application and/or particular roles of the bundled applications. Actions may include some or all of stopping, starting, restarting, taking snapshots, cloning, and rolling back to a prior snapshot. For each action, one or more action hooks may be defined. A hook is a programmable routine that is executed by the orchestration layer when the corresponding action is invoked. A hook may specify a script of commands or configuration parameters input to one or more roles in a particular order. Hooks for an action may include a pre-action hook (executed prior to implementing an action), an action hook (executed to actually implement the action), and a post action hook (executed following implementation of the action). 
     The bundled application  1302  may define a plurality of roles  1312 . Each role may include one or more provisioning constraints. As noted above, the bundled application  1302  and roles  1312  are not aware of the underlying storage nodes  106  and compute nodes  110  inasmuch as these are virtualized by the storage manager  102  and orchestration layer  1300 . Accordingly, any constraints on allocation of hardware resources may be included in the provisioning constraints  1314 . As described in greater detail below, this may include constraints to create separate fault domains in order to implement redundancy and constraints on latency. 
     The role  1312  may define a name space  1316 . A name space  1316  may include variables, functions, services, and the like implemented by a role. In particular, interfaces and services exposed by a role may be included in the name space. The name space may be referenced through the orchestration layer  1300  by an addressing scheme, e.g. &lt;Bundle ID&gt;.&lt;Role ID&gt;.&lt;Name&gt;. In some embodiments, references to the namespace  1316  of another role may be formatted and processed according to the JINJA template engine or some other syntax. Accordingly, each role  1312  may access the variables, functions, services, etc. in the name space  1316  of another role  1312  on order to implement a complex application topology. In some instances, credentials for authorizing access to a role  1312  may be shared by accessing the namespace  1316  of that role. 
     A role  1312  may further include various configuration parameters  1318  defined by the role, i.e. as defined by the developer that created the executable for the role. As noted above, these parameters  1318  may be set by the orchestration layer  1300  according to the static or dynamic configuration parameters  1308 . Configuration parameters may also be referenced in the name space  1316  and be accessible (for reading and/or writing) by other roles  1312 . 
     Each role  1312  may include a container  1320  executing an instance  1322  of the application for that role. The container  1320  may be a virtualization container, such as a virtual machine, that defines a context within which the application instance  1322  executes, facilitating starting, stopping, restarting, and other management of the execution of the application instance  1322 . Containers  1320  may include any container technology known in the art such as DOCKER, LXC, LCS, KVM, or the like. In a particular bundled application  1302 , there may be containers  1320  of multiple different types in order to take advantage of a particular container&#39;s capabilities to execute a particular role  1312 . For example, one role  1312  of a bundled application  1302  may execute a DOCKER container  1320  and another role  1312  of the same bundled application  1302  may execute an LCS container  1320 . 
     Note that a bundled application  1302  as configured in the foregoing description may be instantiated and used or may be saved as a template that can be used and modified later. 
       FIG. 14  illustrates a method  1400  for executing a bundled application  1302  using the orchestration layer  1300 . The method  1400  may include provisioning  1402  storage and computation resources according to the provisioning  1306 . This may include allocating storage volumes according to the storage requirements, assigning the storage volumes to storage nodes  106 , and selecting a compute node  110  or storage node  106  providing the required computational resources (processor cores and memory). 
     The method  1400  may include creating  1404  role instances for the roles  1312  defined by the bundled application  1302 . As described above, this may include creating a container  1320  and instantiating the application instance  1322  of the role  1312  within the container  1320 . The order in which instances  1322  are created and started may be defined in the manifest  1304 . 
     The method  1400  may include configuring  1406  each role according to the configuration parameters  1308 , including executing any included functions to determine values for dynamic parameters. As noted above, starting a bundled application  1302  may further include setting up  1408  the roles  1312  to reference resources in the name space  1316  of another role  1312 . For example, a webserver may be configured to access a database by referencing configuration parameters and services implemented by the database. 
     The method  1400  may further include executing  1410  any hooks  1310  defined for the initial startup of the bundled applications. Accordingly, pre-startup, startup, and post startup hooks may be executed. Some or all of the functions of steps  1402 - 1410  may be defined as part of the pre-startup hook. Other functions may also be performed prior to steps  1402 - 1408  as defined by a pre-startup hook. 
     The actual commencement of execution of the instances  1322  of the bundled application  1302  may be performed in an order specified by the startup hook and may include performing any attendant functions of these instances  1322  as specified by the startup hook. Following startup, one or more other actions may be performed as specified by the developer in the post-startup hook. These actions may invoke functions of the instances  1322  themselves or executed by the orchestration layer  1300  outside of the instances  1322 , such as with respect to an operating system executing the containers  1320  for the instances  1322 . 
     The bundled application  1302  may then be accessed  1412  in order to perform the programmed functionality of the application instances  1322 . As usage occurs, processing resources will be loaded and storage may be filled. The method  1400  may further include adjusting  1414  provisioning according to this usage and may performed adjustment to configuration parameters of the roles  1312  according to this provisioning as defined by the provisioning  1306  and configuration functions  1308 . 
     As noted above, instances of roles may also be created or removed according to usage. Accordingly, where indicate by the manifest  1304 , instances  1322  for a role  1312  may be created according to steps  1402 - 1410  throughout execution of the bundled application  1302  as defined by one or more dynamic functions in the manifest  1304  for that role  1312 . 
     Referring to  FIG. 15 , the illustrated method  1500  may be used to implement provisioning constraints  1314  for a role  1312  or constraints for an entire bundled application  1302 . The method  1500  may be executed by the orchestration layer  1300 , storage manager  102 , or a combination of the two. 
     The method  1500  may include receiving  1502  the provisioning constraint  1314  for one or more roles  1312  of the bundled application  1302  and determining  1504  whether the constraint  1314  specify one or both of a fault domain constraint and a latency constraint. 
     If a latency constraint is found  1506  to be included for a role  1312 , then computational resources and storage resources to be provisioned for the role  1312  may be constrained  1508  to be co-located. In particular, latency may be specified in terms of (a) a minimum network delay, (b) a minimum network throughput, (c) an explicit constraint to place computation and storage resources in the same subnetwork, or (d) an explicit constraint to place computation and storage resources on the same node, i.e. a hybrid compute and storage node  110 ,  106  that performs the functions of both types of nodes with a single computer. 
     This constraint may be used by the orchestration layer to assign computing and storage resources to roles  1312  and storage volumes of the bundled application. For example, one or more storage volumes for the role  1312  will be assigned to storage nodes  106  that can either (a) meet the latency requirement with respect to compute nodes  110  allocated to the role  1312  (b) also provide the computational resources required for the role  1312 . 
     The orchestration layer  1300  may include a resource manager in that accounts for all of the compute storage requirements and constraints and creates a resource allocation plan. This plan describes the virtual nodes (containers  1320 ) that make up the bundled application  1302 . Each virtual node has allocations of processor cores, memory and storage volumes. The resource manager determines the compute host (compute node  110  or hybrid node) for each virtual node and a set of devices for each storage volume of the virtual node. The orchestration layer  1300  sends this mapping of the storage volumes to physical devices to the storage manager  102 , which implements the storage allocation. 
     If the constraint for a role  1312  is found  1510  to include a fault domain constraint, then storage volumes for the role  1312  may be distributed  1512  among the storage nodes  106  of the distributed storage system  100  according to this requirement. For example, if storage volume B is a redundant (e.g., replica or backup copy) of storage volume A, the fault domain constraint may indicate this fact. Accordingly, the storage manager  102  may assign storage volume B to a different storage node  106  than storage volume A. Various degrees of constraint may be specified. For example, a fault domain constraint may simply require a different storage device  108  but not require a different storage node  106 . A fault domain constraint may require that storage nodes  106  to which storage volumes are assigned by in separate subnetworks, different geographic locations, or have some other degree of separation. Similar fault domain constraints may be specified for roles  1312 , which may be constrained to execute on different compute nodes  110  in order to provide redundant services and reduce downtime. 
     The provisioning constraints  1502  based on fault domains and/or latency may be combined with one or more other constraints. For example, a performance constraint (IOPs/second) for a storage node may be imposed. Accordingly, only those compute nodes meeting the performance requirement and the fault domain and/or latency requirements will be selected for provisioning. 
     As noted above, provisioning  1306  may define a processing requirement, such as a number of processing cores and an amount of storage for a role. Accordingly, compute nodes  110  may be selected at step  1508  such that both the latency requirement and processing requirement are met. 
     Referring to  FIG. 16 , the illustrated method  1600  may be executed by the orchestration layer  1302  with respect to a bundled application  1302  in order to create a snapshot of the bundled application  1302  that can be later restored (see the method  1700  of  FIG. 17 ). 
     The method  1600  may include flushing  1602  application buffers to disk. In many instances, performance of an application is accelerated by maintaining data in a cache in memory, such that data in the cache is accessed and updated without requiring writing to a disk in many instances, as known in the art. Accordingly, this buffer may be flushed  1602  to disk by writing all valid data (i.e., not outdated due to a subsequent write) in the cache to the storage device  108  to which that data is addressed, e.g., to which the storage volume referenced by the data is assigned. 
     In a like manner, a file system flush may be performed  1604 . Performing a file system flush may include ensuring that all IOPs pending to be performed by the file system have been executed, i.e. written to disk. As for step  1602 , data written to a cache for the file system this is valid may be written to a storage device  108  to which the data is addressed, e.g., to which the storage volume referenced by the data is assigned. 
     The method  1600  may then include freezing  1606  the application instances  1322  of each role  1312 . In particular, inasmuch as each instance  1322  is executing within container  1320 , the containers  1320  for the roles  1312  may be instructed to pause execution of each instance  1322 . This may include stopping execution and saving a state of execution of each instance  1322  (state variables, register contents, program pointers, function stack, etc.). 
     The method  1600  may further include creating  1608  a snapshot of storage volumes provisioned for the bundled application. This may include executing the method  200  of  FIG. 2  or any of the above-described approaches for implementing a snapshot of a storage volume. 
     The method  1600  may further include creating  1610  a topology snapshot for the bundled application  1302 . The topology of an application may include some or all of the following information as constituted at the time of executing step  1610  a listing of the roles  1312 , which may include one or more instances  1322  of the same role  1322 , relationships between application instances  1322  of roles  1312  (name space cross-references, configuration parameters), storage volumes assigned to roles  1312 , or other information that describes the topology of the bundled application  1302 . Applications may create metadata describing their state of operation. This data may also be saved as part of the topology snapshot. 
     After the snapshot is created according to the method  1600 , the application instances may be resumed, with the application itself not suffering any down time in some embodiments. The bundled application  1302  may then continue to operate. If desired, the application may then be rolled back to the snapshot created according to the method  1600 , as described below with respect to  FIG. 17 . 
       FIG. 17  illustrates a method  1700  for rolling back a bundled application  1302  to a snapshot, such as a snapshot created according to the method  1600 . The method  1700  may be executed by one or both of the orchestration layer  1300  and the storage manager  102 . 
     The method  1700  includes receiving  1702  a rollback instruction, such as from an administrator desiring to return to a stable version of the bundled application  1302 . The remaining steps of the method  1300  may be executed in response to the rollback instruction. 
     The method  1700  may include rolling  1704  back storage volumes assigned to the bundled application  1302  to the snapshots created for the snapshot of the bundled application  1302  (e.g., at step  1608  of the method  1600 ). This may include executing the method  1000  of  FIG. 10  or performing any other approach for rolling back a storage volume to a prior state. 
     The method  1700  may include restoring  1706  application instances from the application snapshot. As described above with respect to step  1606  of the method  1600 , an application instance  1322  may be frozen. Accordingly, data describing a state of execution of the application instance  1322  may be reloaded into a container  1302  for that instance. If needed, the container for that application instance  1322  may be created and the instance  1322  loaded into it prior to loading the state of execution. This is particularly the case where the number of application instances has changed since the application snapshot was created. 
     The method  1700  may further include restoring  1708  the application topology saved for the bundled application at step  1610 . Accordingly, relationships between application instances  1322  of roles  1312  (name space cross-references, configuration parameters), storage volumes assigned to roles  1312 , or other information that describes the topology of the bundled application  1302  may be restored as it was at the time the application snapshot was created 
     The method  1700  further include executing  1710 ,  1712 ,  1714  a pre-restart hook, restart hook, and post restart hook defined for the bundled application. As described above, each hook may be a routine defined by a developer to be executed for a particular action, restarting in this case. In step  1712 , execution of the instances  1322  for the roles  1322  may be restarted, along with any other actions specified by the developer in the restart hook. 
     The bundled application  1302  as restored at steps  1704 - 1714  may then be accessed  1716  as defined by the programming of the application instances and the restored application topology. 
     Note that the snapshot of the bundled application  1302  may be restarted on different storage and compute nodes  106 ,  110  than those on which the bundled application  1302  was executing when the snapshot was created. Accordingly, the application snapshot may be restarted as a clone of the bundled application  1302  or moved to different hardware when executing the method  1700 . 
     In some instances, the hooks of steps  1710 ,  1712 ,  1714  may be different when the application snapshot is being restarted as a clone as desired by a developer. For example, a developer may desire to scale the clone application to increase or decrease a number of databases, number of partitions of a database, or other aspect of the clone application. Accordingly, the hooks of steps  1710 ,  1712 ,  1714  may implement routines to implement this increase or decrease. 
     For example, some applications are able to automatically detect the number of partitions of a database. In such instances, some or all of the hooks  1710 ,  1712 ,  1714  may reduce the number of partitions in a database of the clone applications and rely on the application to discover this change. In other instances, some or all of the hooks  1710 ,  1712 ,  1714  may be programmed to configure an application to access the database with the reduced number of partitions where the application is unable to configure itself. 
     Referring to  FIG. 18 , as noted above, containers  1320  may be implemented as DOCKER containers. However, DOCKER containers are not particularly suited for implementing stateful applications in which some or all of the state of an application is stored in persistent storage. This may be a disadvantage, particularly where a snapshot of an application is to be create and used for rolling back or cloning (see discussion of  FIG. 17 ). 
     In the illustrated approach, a DOCKER container  1320  is modified to use an external graph driver plugin for storing persistent data. In the illustrated embodiment, the graph driver plugin implements a layered file system  1800 . In the illustrated implementation, the layered file system includes various layers  1802   a - 1802   c  that are combined with one another to define a file system as known in the art of graph driver plugins for use with DOCKER containers. In the illustrated embodiment, only one layer  1802   a  is a read/write (R/W) layer and the remaining layers are read only layers. The R/W layer  1802   a  may be configured to mount a remote storage volume  1804  implemented by a storage node  106  according to the methods described herein (see, e.g.,  FIGS. 1 through 7 ). As described above, the storage volume  1804  may be a virtualized storage volume that is implemented without the container  1320  having data regarding a storage node  106  or device  108  on which the storage volume is actually stored. 
     In this manner, any persistent data written or changed by an application instance  1322  executed by the container  1320  will be performed on the remote storage volume  1804 . Accordingly, when a snapshot of the container  1320  is made or the container is moved to a different location, the persistent data may be copied or recreated using the remote storage volume. No tracking of changes or other awareness of the persistent state of the application instance  1322  is required in order to achieve this functionality due to the use of the remote storage volume  1804  to implement the R/W layer  1802   a.    
       FIG. 19  illustrates a method  1900  for using the architecture shown in  FIG. 18 . The method  1900  may be executed on a compute node  110  or hybrid node. The method  1900  may be executed as part of deployment of a bundled application  1300  in order to create and start a container  1320  on the compute node  110 . 
     The method  1900  may include creating  1902  a container  1320 , e.g. a DOCKER container, on the compute node  110  and creating  1904  a layered file system, such as by associating a graph driver plugin with the container  1320 . A remote storage volume may also be created  1906 , as described above with respect to  FIGS. 1 through 7 . Creating  1906  a storage volume may be performed by requesting allocation of a storage volume by the storage manager  102 . 
     The method  1900  may include modifying  1908  metadata of the layered file system to refer to the remote storage volume as layer 0 (the R/W layer) of the layered file system. 
     An instance  1322  of an application executable may be loaded  1910  into the container  1320  as well. The application instance  1322  may be executed  1912 , which may result in writing  1914  of persistent date data for the application instance  1322 . These writes will be routed by the graph driver plugin to the remote storage volume and persistently stored therein. 
     If a move instruction is found  1916  to have been received, the method  1900  may include instantiating  1918  a new container at a new location, e.g., a different compute node. The container may be loaded with an instance  1322  of the executable application. The method  1900  may further include mounting  1920  the remote storage volume from step  1906  to the new container as layer 0 of the layered file system. This may include modifying the metadata for the new container as described above with respect step  1908 . The state of the application instance  1322  may therefore be created using the data in the remote storage volume. 
     In some embodiments, the container to be moved may be frozen and copied to the new location, rather than creating a new container. In that case, a clone of the remote storage volume storing the persistent state data may be mounted to create a clone of the container. 
     The move instruction of step  1916  may be an instruction to move the application instance or be part of a process of cloning the application instance. In either case, execution of the move may be proceeded with creating a snapshot of the application as described above with respect to  FIG. 16 . Likewise, steps  1918  and  1920  may be executed as part of the rollback process of  FIG. 17 . 
     Referring to  FIG. 20 , a bundled application  1302  may be tested to determine robustness in response to failures. For example, a bundled  1302  may include containers  1320  hosting multiple instances  1322  of individual roles  1312  and various replicas of storage volumes in order to provide a degree of redundancy. Accordingly, the method  2000  may be executed by the orchestration layer  1300  to test the ability of this redundancy to handle faults. 
     The method  2000  may include specifying  2002  a fault tolerance. This step may be performed manually as a user or developer of a bundled application  1302  specifies a degree of fault tolerance. The fault tolerance may be specified to the orchestration layer  1300  as a number of failures for one or more entity classes, where the entity classes include some or all of storage nodes, compute nodes, containers, application instances, storage server racks, data centers, a network connection or network component (router, switch, network cable, etc.), or any other component of a bundled application  1302  or the distributed computing system on which the bundled application  1302  is executing. For example, the specification may read as follows: 
     3 compute nodes; 
     2 storage nodes; 
     5 containers; 
     1 rack; and 
     1 switch. 
     The method  2000  may further include collecting  2004  statuses of components of an application. The statuses may include statuses of containers  1320 , application instances  1322 , storage volumes, storage nodes  106 , compute nodes  110 , and possibly other parts of the distributed computing system, such as network components. 
     For example, some or all of these components may report their statuses periodically to the orchestration layer  1300  or the orchestration layer may query these components and evaluate response received to determine whether the response includes an error message. The orchestration layer  1300  may interpret a failure to receive a response to a query from a component as a failure. The orchestration layer  1300  may store these statuses, i.e. a listing including a component and its corresponding status for some or all of the components evaluated. 
     The method  2000  may further include selecting  2006  a fault and selecting  2008  a target for the fault. Selecting  2006  a fault may include selecting an entity class to experience a fault. Selecting a target may include selecting a specific instance of that entity class to experience a fault. Accordingly, if a storage volume is selected, a specific storage volume provisioned for the bundled application may be selected as a target. The selections of steps  2006  and  2008  may be performed randomly, e.g. according to some pseudo random function or using a random number generator as known in the art. 
     The method  2000  may further include making  2010  a snapshot of the target if possible. For example, if the target is a container, a snapshot of the container may be created (see, e.g. the approach for saving and restoring the state of a container  1320 ,  FIGS. 18 and 19 ). If the target is a storage volume, then a snapshot of the storage volume may be created (see, e.g.  FIG. 2 ). Where the target is a compute node  110  or hybrid node, then snapshots of the containers  1320  hosted thereon may be created. When the target is a storage node  106  or hybrid node, snapshots of storage volumes implemented thereby may be created. 
     The method  2000  may include inducing  2012  a fault in the selected target. For example, software implementing a container  1320  or storage volume may include a setting that may be invoked by the orchestration layer to stop it or otherwise cause it to case to perform its function. Likewise, an other element, such as a network component selected as target may be shut down or otherwise instructed to cease functioning. Where a node is selected as the target, then all entities implemented by the node (storage volume and/or container  1320 ) may be instructed to stop or otherwise cease functioning. The node itself may also be instructed to temporarily cease functioning, such as by ceasing to acknowledge or otherwise process network traffic. Where a rack is selected, then each node of the rack may be processed in a similar manner—hosted storage volume and/or containers  1320  stopped and the node itself instructed to cease functioning. For each storage volume and/or container  1320  that is stopped, snapshots may be created prior to stopping. 
     The method  2000  may include evaluating  2014  the state of the bundled application  1302 . This may include evaluating a status as reported by a component of the application  1302 , attempting to access a service implemented by the application  1302 , or any other approach known in the art for evaluating the function of an application. For example, where an interface is exposed by the application  1302 , the orchestration layer  1300  may access a service implemented by that interface. 
     If the state of the bundled application  1302  indicates that it is no longer functioning, then the method  2000  may end. A notification may be presented to the user, such as a message indicating circumstances of the test of steps  2006 - 2012 . For example, the notification may include information such as the entity class and target entity and information regarding the state of the application  1302 , such as error logs generated by the application  1302 . 
     If the state from step  2014  indicates that the application continues to function, then the method  2000  may include incrementing  2016  a fault count and rolling back  2018  the targets to the snapshots created at step  2010 . The fault count may be set to 1 during a first iteration, such that only one target is selected and processed according to steps  2010 - 2014 . 
     The method  2000  may then continue at step  2004  with the number of faults being equal to the fault count. In particular, steps  2006 - 2008  may be performed such that N targets are selected, where N is the fault count. For example, N entity class selections may be made randomly made such that the number of times a particular entity class is selected is less than or equal to the number of faults for that entity class specified at step  2002 . For the M (M=1 or more) times an entity class is selected, M different targets for that entity class may be randomly selected at step  2008 . The method  2000  may then continue repeating at step  2010  with the targets as selected at step  2008 . 
     The method  2000  may continue until one of (a) the application is found to cease to function at step  2014  and (b) the number of faults selected at step  2006  for each entity class is equal to the number specified for each entity class in the fault tolerance. Where condition (b) is met, the application may be determined to meet the fault tolerance and a notification may be transmitted to a user or otherwise recorded that indicates that the fault tolerance of step  2002  is met. In some embodiments, the method  2000  may continue to execute periodically in order to verify that the fault tolerance continues to be satisfied. 
     Referring to  FIG. 21 , a node  106 , such as a storage node or hybrid node, has a plurality of storage devices  108   a - 108   b  mounted thereto, the storage devices  108   a - 108   b  being hard disk drives (HDD), solid state drives (SSD), cloud storage, or some other type of storage device. Each device  108   a - 108   b  stores one or more storage volumes  2100   a  or one or more slices of one or more storage volumes  2100   a ,  2100   b , such as according to the approach described herein above. In particular, as described above, slices may be assigned individually to devices  108   a ,  108   b  such that an entire storage volume  2100   a ,  2100   b  need not reside on the same device  108   a ,  108   b  or even devices mounted to the same node  106 . 
     For each device  108   a ,  108   b , the node  106  may collect usage statistics. For example, a software component implementing disk virtualization in coordination with the storage manager  102  may track IOPs and usage of the storage volumes  2100   a - 2100   b  and/or slices of storage volumes  2100   a ,  2100   b . In particular, storage usage  2102  may indicate the amount of actual data stored in a storage volume  2100   a ,  2100   b  or slice of a storage volume  2100   a ,  2100   b , such as in the form of a number of allocated segments. IOP usage  2104  may indicate a number of IOPs addressed to a storage volume  2100   a ,  2100   b  or slice of a storage volume  2100   a ,  2100   b . IOP usage may track one or both of write IOPs and read IOPs and may track a total number of IOPs per unit time. 
       FIG. 22  illustrates a method  2200  for assigning storage volumes to devices  108   a ,  108   b  of a node  106 . The method  2200  may include provisioning  2202  a storage volume  2100   a  for use by a bundled application  1302  according to the methods described above. The storage volume  2100   a  may then be assigned  2204  to a device  108   a  of the node  106 . For example, the node  106  may be notified of the assignment and the assignment to the node  106  and device  108   a  may be recorded in the volume map  300  for the storage volume  2100   a.    
     The method  2200  may further include monitoring IOPs  2206  for the storage volume  2100   a  and evaluating  2208  whether IOP usage is excessive. In particular, this may include comparing the number of IOPs in a given time window, e.g. 10 ms, 100 ms, 1 second, or the like, to an IOP threshold. If this threshold is exceeded, then the IOPs may be determined  2208  to be excessive. The threshold may be static or dynamic. For example, it may be a function of an average number of IOPs per storage volume assigned to the node  106 . The evaluation of step  2208  may evaluate the number of IOPs in the time window for those slices of the storage volume  2100   a  assigned to the device  108   a  separately from the IOPs for slices assigned to other devices  108   b  or nodes. 
     If the usage is found  2208  to be excessive, then another device may be added  2210  to the storage volume  2100   a  and one or more slices of the storage volume may be redistributed  2212 . For example, a load balancing approach may be used. The number of IOPs in a time window for the slices of the volume  2100   a  may be measured (or past measurements are reused). A first set of slices may be assigned to the first device  108   a  and a second set of slices may be assigned to the second device such that the total number of IOPs in the time window for the slices of the first set is approximately equal to the total number of IOPs in the time window for the slices in the second set. “Approximately equal” may mean equal to within a value between the number of IOPs in the time window for the slice with the highest number of IOPs in the time window and the number of IOPs in the time window for the slice with the lowest number of IOPs in the time window. 
     Steps  2206 - 2212  may be performed periodically such that IOPs are monitored  2206  for a next time window after (and possibly overlapping) the window used at step  2206  of a previous iteration. Subsequent iterations may result in addition of devices or further redistributing  2212  of slices based on excess usage. In some instances, redistribution  2212  may be performed during an iteration of steps  2206 - 2212  without adding  2210  a device, such as when neither device  108   a ,  108   b  is found to be being used within a threshold percentage of its IOPs capacity within the time window of step  2206 . 
     Referring to  FIG. 23 , the illustrated method  2300  may be performed by the node  106  and/or the orchestration layer  1300  in order to adjust the storage available for a bundled application on the node  106 . 
     The method  2300  may include monitoring  2302  storage usage of the storage volumes hosted by the node  106 . For example, storage usage may include counting, by an agent implementing the storage scheme described herein, the number of segments allocated to each storage volume  2100   a ,  2100   b  and/or slice of each storage volume  2100   a ,  2100   b . For example, this information may be obtained from the segment map  314 . 
     The method  2300  may further include estimating  2304  a fill rate for the storage volumes  2100   a ,  2100   b  on the node  106 , which may include the fill rate for the set of slices of a particular storage volume  2100   a ,  2100   b  on a particular device  108   a ,  108   b . The method  2300  may be performed for multiple storage volumes separately (“the subject volume”). In particular, a rate of write IOP generation, segment allocation, or other metric of storage increase per unit time on the node  106  for the subject volume within a predetermined time window may be calculated. The fill rate for the subject volume may be evaluated with respect to some or all of (a) an amount of unused storage in the subject volume, (b) an amount of unused storage in slices of the subject volume assigned to the node  106 , (c) an amount of unused storage on an individual device  108   a ,  108   b , and (d) a total amount of unused storage on all devices  108   a ,  108   b  mounted to the storage node  106 . Using these values, step  2304  may further include estimating a time until full as one or more of the values of (a)-(d) divided individually by the fill rate. 
     The method  2300  may include evaluating  2306  whether more storage is needed for the subject node. In particular, if a time until full according to or more of values (a)-(d) is below a threshold time, it may be determined  2306  that more storage is needed for the subject volume. 
     If no storage is found  2306  to be needed, then the method  2300  may end and be repeated at a later time, such as according to a predefined repetition period. 
     If more storage is found  2306  to be needed, the method  2300  may further include evaluating  2308  whether more memory, processors, or other computing resources are needed. In particular, high usage of storage may be accompanied by additional requirements for processing, memory, network bandwidth, or other resources. 
     Accordingly, step  2300  may include evaluating current (e.g., measured during a time window defined for measuring) memory usage, processor usage, network bandwidth usage, NIC usage (network interface controller), rack usage (e.g., number of rack blades in use and amount of use of each blade), or other usage statistic. For example, where the node  106  is a hybrid node, then these statistics may be measured to characterize processing needs of one or more containers hosted by the node  106  and to which the subject volume is mounted. A measured usage value may be compared to a corresponding threshold, which, if exceeded, results in a positive outcome to the evaluation of step  2308 . 
     For example, where step  2308  indicates more processing or memory is needed, the method  2300  may include creating  2310  a new container  1320 , provisioning  2312  a new storage volume, and mounting  2314  the new storage volume to the new container  1320 . In particular, the new container may be loaded with an instance  1322  of the same application as is accessing the subject volume. In this manner, IOPs may be distributed across multiple containers  1320  and multiple storage volumes thereby resolving the need for more storage and more processors and/or memory. In a like manner, provisioning a new container and storage volume at a different location in a distributed computing system may also eliminate bottlenecks for network traffic determined to be present at step  2308 . 
     Where steps  2310 - 2314  are executed, the orchestration layer  1300  may notify the bundled application  1302  of the available new container and configure the bundled application  1302  to use the new container, such as by executing a hook  1310  that performs these functions as specified by a developer of the application  1302 . 
     If more storage is found  2306  to be needed but more processing, memory, or other resources are not found  2308  to be needed, the method  2300  may evaluate one or more alternatives to handle the need for additional storage. 
     For example, the method  2300  may include evaluating  2316  whether expanding of the subject volume, i.e., increasing its size, is possible and desirable. For example, if a device  108   a - 108   b  has unused storage capacity, the size of the subject volume may be increased  2318  to use up some or all of this capacity, such that the amount of unused storage capacity is above some threshold for excess capacity. 
     In some embodiments, step  2316  may include evaluating usage of the application&#39;s  1302  use of the subject volume. For example, where growth is slow and IOPs are nearly balanced between read and write IOPs, growing of the subject volume may be determined to be a suitable alternative. 
     If expanding is found  2316  not to be possible, the method  2300  may include evaluating  2320  whether performing garbage collection (GC) on the subject volume would resolve the lack of storage. For example, step  2320  may include evaluating some or all of the following: (a) an elapsed time since GC was performed on the subject volume, (b) an amount of invalid data in the subject volume, (c) and amount of valid data in the subject volume. Determining the amount of valid and invalid data in a slice of a storage volume may be performed as describe above (see description of  FIG. 6 ). Values according to (a) or (c) may be compared to a corresponding threshold condition, which, if met, may invoke performing  2322  GC. For example, if the elapsed time is greater than an elapsed time, then GC may be performed. If the amount of invalid data is above an invalid data threshold, GC may be performed in some embodiments. If the amount of valid data is below a valid data threshold, GC may be performed in some embodiments. 
     The method  2300  may further include evaluating  2324  whether adding a disk is needed. In some embodiments, if neither of steps  2316 - 2320  are found to indicate other options for increasing storage, adding  2324  of a disk is found  2324  to be needed. In other cases, additional considerations may be evaluated at step  2324 , such as whether an additional disk is mounted to the node  106  or is available for mounting to the node, such as based on an inventory or topology of a network as provided to the node  106  or orchestration layer. If addition of a disk to the subject volume is determined  2324  to be possible and desirable, the method  2300  may include adding  2326  an additional disk to the subject volume and redistributing  2328  slices of the subject volume, such as in the manner described for steps  2210  and  2212  of the method  2200 . 
     The method  2300  may be repeated periodically in order to accommodate changes in usage. 
     Referring to  FIG. 24 , In some embodiments, the node  106  may additionally maintain a volume limit  2400   a ,  2400   b  for each device  108   a ,  108   b  that specifies the number of storage volumes  23100   a - 2100   c  that may be allocated to that device  108   a ,  108   b . The limit may be specified as a number of volumes, a number of slices, a maximum quantity of storage represented by allocated storage volumes (GB, TB, etc.). The limits  2400   a ,  2400   b  may be set initially by the orchestration layer  1300  and may subsequently be adjusted by logic executing on the node  106  or orchestration layer  1300  as described with respect to  FIG. 25 . 
       FIG. 25  illustrates a method  2500  for adjusting the volume limit for devices  108   a ,  108   b  of a node  106  and which may be executed by the node  106  and/or orchestration layer  1300 . 
     The method  2500  may include setting  2502  an initial volume limit. This may be a system-wide default implemented by the orchestration layer  1300  or an application-wide limit specified by the bundled application  1302 . The volume limit may be specified by a bundled application  1302  for each role  1312 . The volume limit may be distributed by the orchestration layer  1300  to the nodes  106  to which storage devices  108   a ,  108   b  are mounted. 
     The method  2500  may further include monitoring  2504  throughput (IOPs) of the storage volumes  2100   a ,  2100   b  or slices of these volumes. In particular, the number of IOPs per unit time (e.g., per 10 ms, 100 ms, 1 second, or other period) may be measured periodically. Read and write IOPs may be counted separately or aggregated. 
     The method  2500  may further include evaluating  2506  whether there is a throughput imbalance on the device  108   a ,  108   b  of the node  106 . Evaluating throughput may include evaluating read and write IOPs and may also include evaluating IOPs from performing garbage collection (GC), replication, or other sources of IOPs. For example, if the aggregate throughput of the volumes or slices of volumes on a device  108   a ,  108   b  may be determined to be imbalanced based on one or more of the following criteria:
         1. The aggregate throughput is above a predetermined upper threshold for the device  108   a ,  108   b.      2. The aggregate throughput is below a predetermined lower threshold for the device  108   a ,  108   b.      3. The aggregate throughput of a first device  108   a ,  108   b  is above the throughput of a second device  108   b ,  108   a  of the node by some relative amount, e.g. T 1  is greater than X*T 2 , where T 1  is the throughput of the first device, T 2  is the throughput of the second device, and X is a value greater than 1.   4. The aggregate throughput of a first device  108   a ,  108   b  is above the throughput of a second device  108   b ,  108   a  of the node by some relative amount, e.g. T 1  is greater than Y+T 2 , where T 1  is the throughput of the first device, T 2  is the throughput of the second device, and Y is a predetermined number of IOPs per unit time.       

     If an imbalance is found  2506 , the method  2500  may include reducing  2508  the volume limit for the device  108   a ,  108   b  having high throughput according to conditions 1, 3, or 4, above. For example, if the volume limit is 10, only two volumes (or some number of slices of volumes) have been assigned to device  108   a , and its throughput is high enough to meet one of the conditions 1, 3, or 4, the volume limit for device  108   a  may be reduced, such as to a limit of two. 
     If an imbalance is found, the method  2500  may further include augmenting  2510  the volume limit for a device  108   a ,  108   b  that has low throughput according to condition 2 or is the second device where a first device meets condition 3 or 4. For example, a device  108   b  that has a number of volumes assigned thereto meeting the volume limit may have its volume limit increased to permit the assignment of more storage volumes or slices inasmuch as its throughput is low. 
     Note that in some instances only step  2508  is executed where an imbalance found  2506 . In other instances, both of steps  2508   a  and  2510  are executed. In still others only step  2510  is performed. For example, if condition 1 is met, only step  2508  is performed in some embodiments. If condition 2 is met, only step  2510  is performed in some embodiments. 
     The method  2500  may further include evaluating  2512  whether a new device has been added to the node  106 . In that case, the method  2500  may include rebalancing volume limits according to usage. For example, the volume limit of a first device having higher throughput relative to a second device of the node may be reduced in response to addition of a third device such that additional volumes will be assigned to the third device. Where the rebalancing of the load limits causes the volume limit of the first device to be less than the number of volumes assigned to it, one or more volumes assigned to the first device may be transferred to the third device. This transfer may be based on usage. For example, volumes may be transferred to the third device based on throughput with the lowest throughput volumes being transferred until the volume limit is met on the first device. 
     In the event that a new storage volume is found  2514  to be added to the node  106 , the storage volume maybe assigned  2516  to a device based on the current load limits as adjusted according to any of the foregoing steps and throughput of the devices. For example, the new volume may be assigned to the device having the lowest throughput of those devices having volumes assigned thereto under their volume limits. 
       FIG. 26  is a block diagram illustrating an example computing device  2600 . Computing device  2600  may be used to perform various procedures, such as those discussed herein. The storage manager  102 , storage nodes  106 , compute nodes  110 , and hybrid nodes, may have some or all of the attributes of the computing device  2600 . 
     Computing device  2600  includes one or more processor(s)  2602 , one or more memory device(s)  2604 , one or more interface(s)  2606 , one or more mass storage device(s)  2608 , one or more Input/output (I/O) device(s)  2610 , and a display device  2630  all of which are coupled to a bus  2612 . Processor(s)  2602  include one or more processors or controllers that execute instructions stored in memory device(s)  2604  and/or mass storage device(s)  2608 . Processor(s)  2602  may also include various types of computer-readable media, such as cache memory. 
     Memory device(s)  2604  include various computer-readable media, such as volatile memory (e.g., random access memory (RAM)  2614 ) and/or nonvolatile memory (e.g., read-only memory (ROM)  2616 ). Memory device(s)  2604  may also include rewritable ROM, such as Flash memory. 
     Mass storage device(s)  2608  include various computer readable media, such as magnetic tapes, magnetic disks, optical disks, solid-state memory (e.g., Flash memory), and so forth. As shown in  FIG. 26 , a particular mass storage device is a hard disk drive  2624 . Various drives may also be included in mass storage device(s)  2608  to enable reading from and/or writing to the various computer readable media. Mass storage device(s)  2608  include removable media  2626  and/or non-removable media. 
     I/O device(s)  2610  include various devices that allow data and/or other information to be input to or retrieved from computing device  2600 . Example I/O device(s)  2610  include cursor control devices, keyboards, keypads, microphones, monitors or other display devices, speakers, printers, network interface cards, modems, lenses, CCDs or other image capture devices, and the like. 
     Display device  2630  includes any type of device capable of displaying information to one or more users of computing device  2600 . Examples of display device  2630  include a monitor, display terminal, video projection device, and the like. 
     Interface(s)  2606  include various interfaces that allow computing device  2600  to interact with other systems, devices, or computing environments. Example interface(s)  2606  include any number of different network interfaces  2620 , such as interfaces to local area networks (LANs), wide area networks (WANs), wireless networks, and the Internet. Other interface(s) include user interface  2618  and peripheral device interface  2622 . The interface(s)  2606  may also include one or more peripheral interfaces such as interfaces for printers, pointing devices (mice, track pad, etc.), keyboards, and the like. 
     Bus  2612  allows processor(s)  2602 , memory device(s)  2604 , interface(s)  2606 , mass storage device(s)  2608 , I/O device(s)  2610 , and display device  2630  to communicate with one another, as well as other devices or components coupled to bus  2612 . Bus  2612  represents one or more of several types of bus structures, such as a system bus, PCI bus, IEEE 1394 bus, USB bus, and so forth. 
     For purposes of illustration, programs and other executable program components are shown herein as discrete blocks, although it is understood that such programs and components may reside at various times in different storage components of computing device  2600 , and are executed by processor(s)  2602 . Alternatively, the systems and procedures described herein can be implemented in hardware, or a combination of hardware, software, and/or firmware. For example, one or more application specific integrated circuits (ASICs) can be programmed to carry out one or more of the systems and procedures described herein. 
     In the above disclosure, reference has been made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific implementations in which the disclosure may be practiced. It is understood that other implementations may be utilized and structural changes may be made without departing from the scope of the present disclosure. References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Implementations of the systems, devices, and methods disclosed herein may comprise or utilize a special purpose or general-purpose computer including computer hardware, such as, for example, one or more processors and system memory, as discussed herein. Implementations within the scope of the present disclosure may also include physical and other computer-readable media for carrying or storing computer-executable instructions and/or data structures. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer system. Computer-readable media that store computer-executable instructions are computer storage media (devices). Computer-readable media that carry computer-executable instructions are transmission media. Thus, by way of example, and not limitation, implementations of the disclosure can comprise at least two distinctly different kinds of computer-readable media: computer storage media (devices) and transmission media. 
     Computer storage media (devices) includes RAM, ROM, EEPROM, CD-ROM, solid state drives (“SSDs”) (e.g., based on RAM), Flash memory, phase-change memory (“PCM”), other types of memory, other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. 
     An implementation of the devices, systems, and methods disclosed herein may communicate over a computer network. A “network” is defined as one or more data links that enable the transport of electronic data between computer systems and/or modules and/or other electronic devices. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a computer, the computer properly views the connection as a transmission medium. Transmissions media can include a network and/or data links, which can be used to carry desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. Combinations of the above should also be included within the scope of computer-readable media. 
     Computer-executable instructions comprise, for example, instructions and data which, when executed at a processor, cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. The computer executable instructions may be, for example, binaries, intermediate format instructions such as assembly language, or even source code. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the described features or acts described above. Rather, the described features and acts are disclosed as example forms of implementing the claims. 
     Those skilled in the art will appreciate that the disclosure may be practiced in network computing environments with many types of computer system configurations, including, an in-dash vehicle computer, personal computers, desktop computers, laptop computers, message processors, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, mobile telephones, PDAs, tablets, pagers, routers, switches, various storage devices, and the like. The disclosure may also be practiced in distributed system environments where local and remote computer systems, which are linked (either by hardwired data links, wireless data links, or by a combination of hardwired and wireless data links) through a network, both perform tasks. In a distributed system environment, program modules may be located in both local and remote memory storage devices. 
     Further, where appropriate, functions described herein can be performed in one or more of: hardware, software, firmware, digital components, or analog components. For example, one or more application specific integrated circuits (ASICs) can be programmed to carry out one or more of the systems and procedures described herein. Certain terms are used throughout the description and claims to refer to particular system components. As one skilled in the art will appreciate, components may be referred to by different names. This document does not intend to distinguish between components that differ in name, but not function. 
     It should be noted that the sensor embodiments discussed above may comprise computer hardware, software, firmware, or any combination thereof to perform at least a portion of their functions. For example, a sensor may include computer code configured to be executed in one or more processors, and may include hardware logic/electrical circuitry controlled by the computer code. These example devices are provided herein purposes of illustration, and are not intended to be limiting. Embodiments of the present disclosure may be implemented in further types of devices, as would be known to persons skilled in the relevant art(s). 
     At least some embodiments of the disclosure have been directed to computer program products comprising such logic (e.g., in the form of software) stored on any computer useable medium. Such software, when executed in one or more data processing devices, causes a device to operate as described herein. 
     While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the disclosure. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Further, it should be noted that any or all of the aforementioned alternate implementations may be used in any combination desired to form additional hybrid implementations of the disclosure.