Patent Publication Number: US-11023328-B2

Title: Redo log for append only storage scheme

Description:
BACKGROUND 
     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 to an original state or some intermediate state. In this manner, changes to software 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 volume in accordance with an embodiment of the present invention; 
         FIG. 13  is a diagram illustrating the block processing of write requests in accordance with an embodiment of the present invention; 
         FIG. 14  is a process flow diagram of a method for block processing write requests in accordance with an embodiment of the present invention; 
         FIG. 15  is a diagram illustrating data structures for implementing a redo log in accordance with an embodiment of the present invention; 
         FIG. 16  is a process flow diagram of a method for using a redo log in accordance with an embodiment of the present invention; 
         FIG. 17  is a process flow diagram of a method for recovering data from a redo log in accordance with an embodiment of the present invention; and 
         FIG. 18  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 preferably 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 an 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  will 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 volume 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 snapshots of a clone volume. 
     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 volume and branch to a node A 1  representing the clone volume. In the illustrated example, a clone instruction was received with respect to the snapshot of node S 2 . This resulted in the creation of a clone volume 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 volume 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 the snapshot of the clone volume 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 volume), 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 volume, 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 snapshot of a clone volume that is a clone of the snapshot represented by node S 4 . Subsequent snapshots of the clone volume are represented by nodes B 1  to B 3 . 
     Referring again to  FIG. 8 , the creation of a snapshot for a clone volume 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 volume on storage nodes  106  storing slices of the cloned storage volume and mapped to the clone volume. IOPs referencing the clone volume may be executed  808 , such as according to the method  400  of  FIG. 4 . 
     In some instances, it may be desirable to store snapshots of a clone volume on a different storage node  106  than the principal snapshots. Accordingly, the method  800  may include allocating  806  segments to the clone volume on the different storage node  106 . This may be invoked by sending a new snapshot instruction referencing the clone volume (i.e., an identifier of the clone volume) to the different storage node  106  and instructing one or more compute nodes  110  to route IOPs for the clone volume 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 volumes. 
     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 snapshot of a clone volume. 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 snapshot of a clone volume, 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 volumes, snapshots of clone volumes, clones of clone volumes and descendent snapshots of any snapshots of any clone volume 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 a snapshot of a clone volume are treated the same as principal snapshots and clones of any of these descendants are treated the same as a snapshot of a clone volume. 
     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. 
       FIG. 13  is a diagram illustrating processing of batches of write requests. On the compute node  110 , a data buffer  1300  stores data payloads  1302  of write requests all addressed to a same storage node  106 . An index  1304  stores the LBAs  1306  from the write requests and may also store the sizes  1308  of the data payloads from each write requests. The data buffer  1300  may store data payloads for a predetermined time period or until the total amount of data in the data buffer  1300  reaches a threshold size. 
     Once the buffer is full or a time limit is reached, the data payloads  1302  are written  1310  in a block write to the storage device  108  of the storage node  106  addressed by the write requests. The data payloads  1302  are then written as data  326  to a physical segment  324 . The physical segment  324  may be the current segment identified or allocated as described above with respect to the method  400 . In particular, the data payloads  1302  may be written to the current segment starting at the data offset  320  for the current segment. Where the block write exceeds the capacity of the current segment, a portion may be written to the current segment and the remainder written to a newly allocated segment. 
     The index  1304  may also be written to memory  1310  of the storage node  106 . For example, the LBAs  1306  and sizes  1308  may be written to an index page buffer  1312 , such as in the form of metadata entries  1314 . When the index page buffer  1312  is full it is written to the current segment in the form of an index page  328 . In particular, the contents of the index page buffer  1312  may be written as an index page  328  at the first free location from the second end of the current segment. The contents of the index page buffer  1312  may be combined with a header to form the index page  328 . Likewise, the contents of the index page may be combined with the physical offset locations within the current segment to which the corresponding payload data  1302  were written. 
       FIG. 14  illustrates a method  1400  for performing block writes that may be executed by a compute node  110  and a storage node  106 . The method  1400  may include, by the compute node  110 , buffering  1402  data payloads by the compute node  110  as well as buffering  1404  metadata for the data payloads, i.e. the LBA and size of each write request. The compute node  110  then transmits  1406  a block write to the storage node. The block write may include both the buffered data payloads as well as the buffered metadata corresponding to the buffered data payloads. 
     The storage node  106  receives the data payloads and metadata. The storage node  106  writes  1408  the data payloads to the current segment. However, the metadata may be buffered  1410  in an index page buffer  1312  and not written to an index page  328  of the current segment. 
     For example, the method  1400  may include evaluating  1412  whether the storage node  106  has any pending write requests, such as any pending write requests for the storage device  108  storing the current segment. If so, then the metadata is not written. At a time when there is found  1412  to be no pending write requests, the index page buffer  1312  may then be written  1414  to the current segment. In this manner, the latency of a write command will be the latency of the block write of the data payload rather than the latency of two writes (payload and metadata). In some embodiments, if the index page buffer  1312  is filled to the size of an index page  328 , it will be written regardless of whether there is a pending write request. In other embodiments, the index page buffer  1312  is large enough that filling of the index page buffer is not likely to occur. In such embodiments, the contents of the index page buffer  1312  may be written to multiple index pages  328  which may span multiple physical segments  324 . 
     Referring to  FIG. 15 , in the storage scheme described above, each write request requires two write operations to the storage device  108 , a first write with the payload data and a second write with metadata written to an index page  328 . This increases the latency of executing write requests. Performing block writes of payload and metadata as described with respect to  FIGS. 13 and 14  helps reduce some of this latency. The approach of  FIG. 15  illustrates an approach for reducing the latency of the two writes to the storage device  108 . 
     In the illustrated approach, a write request  1502  includes payload data  1504  and metadata  1506 . The metadata  1506  may include a volume identifier, slice identifier (e.g., slice offset), LBA within the slice corresponding to the slice identifier, or other data. Although the logical storage unit of a slice of a storage volume is discussed herein, any other definition for a logical storage unit may be used and stored in a like manner. 
     The metadata  1506  may further include data describing an encryption algorithm used to encrypt the payload data, a compression algorithm used to compress the payload data, a type of the write request according to a storage scheme, parity check data, error correction data, or the like. For example, there may be various types of data entries in an index, such as a data entry that represents written data that is to be retained and discard entries that are executed as part of an SSD trim command or a LINUX “block discard” command. 
     The payload data is written to a data buffer  1508  in the memory  1500  of the storage node  106 . The metadata  1506  is used to create an entry in an index buffer  1510  (“index entry”) and an entry in a redo buffer  1412  (“redo entry”). 
     The index entry may include some or all of the same data included in entries of the index pages  328 , as described above, including LBA  332 , physical offset  334 , and possibly a size  336 . As described below, the index entry may be generated and written to the index buffer  1510  before the payload data is written to a physical segment  324 . However, the physical offset  334  at which the payload data will be written may be determined, as described above with respect to  FIG. 4 , and added to the index entry. The index entry may include some or all of the other items of metadata  1506 , such as some or all of the encryption algorithm, compression algorithm, parity check data, type, or the like. 
     Note that the index buffer  1510  may be for a particular segment  324 . That is to say that each index buffer  1510  accumulates data that will be written to the index pages of a specific segment  324 . Accordingly, each index buffer  1510  may be associated with a particular PSID  316  for a particular physical segment  324 . The manner in which data is added to the index buffer  1510  and formatted may be identical to the manner data is added and formatted in the index pages  328  as described above. Alternatively, the index entries may be modified when written to the index pages  328  of a physical segment  324  in order to have the order and formatting as described above with respect to  FIGS. 3 and 4 . 
     The redo entry for a write request may include the index entry for the write request and may further include information describing where the index entry will be written. In some embodiments, the redo buffer  1512  is not specific to a particular segment  324 , storage volume or slice offset within a storage volume. For example, the redo buffer  1512  may accumulate redo entries for all write requests for a particular storage device  108  of a storage node  106 . As a result, redo entries for different segments  324 , different storage volumes, and different slices of storage volumes may be interleaved within the redo buffer  1512 . 
     Accordingly, redo entries may be labeled with data enabling the redo entry to be mapped to a particular storage volume and slice offset within a storage volume. For example, each redo entry may include a PSID  316  of the physical segment  324  to which the index entry of that redo entry will be written. The segment map  314  maps a PSID  316  to a corresponding VSID  318  and slice ID  310  mapped to a volume ID  312  and offset  304  in the slice map (see discussion of  FIG. 3 , above). Accordingly, the VSID  318  mapped to the physical segment  324  may be included in the redo entry instead. In the following disclosure, the PSID is referenced with the understanding that the VSID could also be used using the mapping provided by the segment map  314 . 
     A write request is acknowledged as complete only after the data buffer  1508  has been written to the payload data  1514  of the segment  324  and the redo buffer  1512  has been written to a redo segment  1518 . Note that this still requires two writes to be performed. However, latency may be reduced by issuing the data write and the redo write to the device simultaneously. In prior approaches these writes are serialized. With both writes being performed in parallel and the data write typically being much larger than the metadata write, the total latency will only be the latency of a single write command. 
     When the index buffer  1510  for a segment  324  is full, the index buffer  1510  is written to index storage  1516  reserved for the index buffer  1510  in the segment  324 . The content and format of the index entries as written to the index storage  1516  may be as described above with respect to  FIGS. 3 and 4 . By writing the redo buffer  1512  to the storage device  108 , the index buffer  1510  need not be written to the storage device  108  before acknowledging each write request for which entries are added to the index buffer  1510 , thereby reducing latency. 
     Referring to  FIG. 16 , write requests may be processed by the storage node  106  according to the method  1600 . The method  1600  includes receiving  1602  a write request  1502  and writing  1604  the payload data  1504  of the write request  1502  to the data buffer  1508 . Note that the write request of step  1602  may be a block write request as described above with respect to  FIGS. 13 and 14 . 
     The method  1600  may further include generating  1606  an index entry and writing  1608  the index entry to the index buffer  1510 . In particular, this may include determining the segment  324  and physical offset within the physical segment  324  to which the payload data of the write request is to be written, as described above with respect to  FIGS. 3 and 4 . As also noted above, the index entry may include some or all of the metadata  1506  included in the write request  1502 . 
     The method  1600  may further include writing  1610  a redo entry to the redo buffer  1512 . The content of the redo entry may be as described above with respect to  FIG. 15 . In particular, the redo entry may include the index entry of step  1606  and additionally include an identifier (PSID) of the segment  324  to which the payload data  1504  of the write request will be written. 
     The method  1600  may further include writing  1612  the contents of the data buffer to a segment  324 , i.e., the segment identified at step  1606 . The manner in which this segment  324  is selected and the payload data is written may be as described above with respect to  FIGS. 3 and 4 . Writing  1612  the contents may include performing a block write as described above with respect to  FIGS. 13 and 14 . Note that the write of step  1612  may or may not include writing of the entries in an index page  328  corresponding to the payload data, as will be discussed below. 
     The method  1600  may further include writing  1614  the contents of the redo buffer  1512  to a redo segment  1518 . Steps  1612  and  1614  may be performed in parallel as shown in  FIG. 16 . The redo buffer  1512  may be smaller in size than the redo segment  1518 . Alternatively, the redo buffer  1512  may be the same size as the redo segment  1518 . In any case, the redo buffer  1512  may or may not be full when step  1614  is executed. Accordingly, step  1614  may include writing the contents of the redo buffer  1512  to a redo segment  1518 . If no redo segment is available, the storage node  106  allocates a redo segment  1518 . If the contents of the redo segment buffer  1512  are greater than available space in a redo segment  1518 , another redo segment  1518  may be allocated and the contents may be split among the two redo segments  1518 . 
     As is apparent from the above description, there may be multiple redo segments  1518 . In order to permit parallel processing, there may be multiple redo segments  1518  with space available for writing redo entries. Likewise, the write of step  1612  and the write of step  1614  may be performed in parallel, e.g. added to the queue of a storage device  108  for execution rather than waiting for one of the writes  1612 ,  1614  to complete before adding the other write  1612 ,  1614  to the queue. 
     Only after the writes of both step  1612  and  1614  are completed successfully does the method  1600  include acknowledging  1616  that the write request has completed successfully, such as by transmitting an acknowledgment message to an application on a compute node  110  that generated the write request. 
     If the index buffer  1618  for the segment  324  written to at step  1612  is determined  1618  to be full as a result of the write of step  1608 , then the index buffer is written  1620  to that segment  324  and that segment  324  is finalized  1622 , e.g. a flag or other value is set to indicate that the segment  324  is finalized and will not be added to. 
     The method  1600  may further include evaluating  1624  for each redo segment  1518  written to at step  1614  whether finalizing of the segment at step  1622  means that the each redo segment  1518  stores only redo entries for segments  324  that are finalized. As noted above, each redo entry includes a reference to the PSID  324  of the segment to which it corresponds. If all the PSIDs referenced in the redo segment  1518  are determined to correspond to finalized segments  324 , then the redo segment  1518  is freed  1626 , i.e. marked as free or otherwise made available to be overwritten. 
       FIG. 17  illustrates a method  1700  for using one or more redo segments  1518  to recover from a crash of the storage node  106 . The method  1700  may include detecting  1702  restarting of the storage node  106 . In response, the remaining steps of the method  1700  may be executed. 
     The method  1700  may include evaluating  1704  whether any redo segments  1518  are stored in the storage device  108 , whether any redo segments  1518  have been allocated but not subsequently freed at step  1626 . If not, no further action is required, since there are no redo segments  1518  including redo entries for non-finalized segments  324 . 
     If so, then the method  1700  may include reading  1706  the redo segments  1518  and identifying the PSIDs referenced. For each PSID referenced, the index entries included in the redo entries referencing that PSID are retrieved and written  1708  to an index buffer  1510 . As described above with respect to  FIGS. 3 and 4 , the index pages  328  and the entries thereof may be written in order such that it is possible to identify the index entry corresponding to the last received-write request. 
     Accordingly, the ordering may be determined from the redo segments  1518 . For example, where redo entries may be added to redo segments  1518  in the order received and redo segments  1518  may be assigned indexes in the order of allocation. Accordingly, the higher the redo segment index and the closer to the end of the redo segment  1418 , the later the index entry for that redo entry was created. Accordingly, the order of creation of the index entries may be determined from the redo entries and the index entries may be added to the index buffer  1510  according to the order of creation. 
     If the index buffer  1510  for a PSID is full, the index buffer is written  1710  to the index storage  1516  of the segment  324  corresponding to that PSID and that segment  324  is finalized. If all the PSIDs referenced in a redo segment  1518  are finalized, then that redo segment  1518  is freed  1710 , as described above with respect to  FIG. 16 . If an index buffer  1510  is not full, then its corresponding segment  324  is not finalized and the redo segment referencing the PSID of the segment  324  is not freed  1710 . 
     After restarting, processing may continue for new write requests as described above with respect to  FIG. 16 . 
       FIG. 18  is a block diagram illustrating an example computing device  1800 . Computing device  1800  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, or any computing device referenced herein may have some or all of the attributes of the computing device  1800 . 
     Computing device  1800  includes one or more processor(s)  1802 , one or more memory device(s)  1804 , one or more interface(s)  1806 , one or more mass storage device(s)  1808 , one or more Input/output (I/O) device(s)  1810 , and a display device  1830  all of which are coupled to a bus  1812 . Processor(s)  1802  include one or more processors or controllers that execute instructions stored in memory device(s)  1804  and/or mass storage device(s)  1808 . Processor(s)  1802  may also include various types of computer-readable media, such as cache memory. 
     Memory device(s)  1804  include various computer-readable media, such as volatile memory (e.g., random access memory (RAM)  1814 ) and/or nonvolatile memory (e.g., read-only memory (ROM)  1816 ). Memory device(s)  1804  may also include rewritable ROM, such as Flash memory. 
     Mass storage device(s)  1808  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. 18 , a particular mass storage device is a hard disk drive  1824 . Various drives may also be included in mass storage device(s)  1808  to enable reading from and/or writing to the various computer readable media. Mass storage device(s)  1808  include removable media  1826  and/or non-removable media. 
     I/O device(s)  1810  include various devices that allow data and/or other information to be input to or retrieved from computing device  1800 . Example I/O device(s)  1810  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  1830  includes any type of device capable of displaying information to one or more users of computing device  1800 . Examples of display device  1830  include a monitor, display terminal, video projection device, and the like. 
     Interface(s)  1806  include various interfaces that allow computing device  1800  to interact with other systems, devices, or computing environments. Example interface(s)  1806  include any number of different network interfaces  1820 , such as interfaces to local area networks (LANs), wide area networks (WANs), wireless networks, and the Internet. Other interface(s) include user interface  1818  and peripheral device interface  1822 . The interface(s)  1806  may also include one or more peripheral interfaces such as interfaces for printers, pointing devices (mice, track pad, etc.), keyboards, and the like. 
     Bus  1812  allows processor(s)  1802 , memory device(s)  1804 , interface(s)  1806 , mass storage device(s)  1808 , I/O device(s)  1810 , and display device  1830  to communicate with one another, as well as other devices or components coupled to bus  1812 . Bus  1812  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  1800 , and are executed by processor(s)  1802 . 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.