Patent Publication Number: US-11385799-B2

Title: Storage nodes supporting multiple erasure coding schemes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/745,151, filed on Jan. 16, 2020, which is a continuation of U.S. patent application Ser. No. 15/789,859, filed on Oct. 20, 2017, which is a continuation of U.S. patent application Ser. No. 15/180,787, filed on Jun. 13, 2016, which is a continuation of U.S. patent application Ser. No. 14/296,167, filed on Jun. 4, 2014, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     Solid-state memory, such as flash, is currently in use in solid-state drives (SSD) to augment or replace conventional hard disk drives (HDD), writable CD (compact disk) or writable DVD (digital versatile disk) drives, collectively known as spinning media, and tape drives, for storage of large amounts of data. Flash and other solid-state memories have characteristics that differ from spinning media. Yet, many solid-state drives are designed to conform to hard disk drive standards for compatibility reasons, which makes it difficult to provide enhanced features or take advantage of unique aspects of flash and other solid-state memory. 
     It is within this context that the embodiments arise. 
     SUMMARY 
     In some embodiments, a plurality of storage nodes in a single chassis is provided. Each of the plurality of storage nodes has a storage capacity with nonvolatile solid-state memory for user data storage. The plurality of storage nodes is configured to support uniform storage capacities and non-uniform storage capacities among the plurality of storage nodes, as a storage cluster. The plurality of storage nodes is configured to distribute the user data and metadata throughout the plurality of storage nodes such that the plurality of storage nodes can read the user data, using erasure coding, despite loss of two of the plurality of storage nodes. 
     Other aspects and advantages of the embodiments will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. These drawings in no way limit any changes in form and detail that may be made to the described embodiments by one skilled in the art without departing from the spirit and scope of the described embodiments. 
         FIG. 1  is a perspective view of a storage cluster with multiple storage nodes and internal storage coupled to each storage node to provide network attached storage, in accordance with some embodiments. 
         FIG. 2  is a system diagram of an enterprise computing system, which can use one or more of the storage clusters of  FIG. 1  as a storage resource in some embodiments. 
         FIG. 3  is a block diagram showing multiple storage nodes and non-volatile solid state storage with differing capacities, suitable for use in the storage cluster of  FIG. 1  in accordance with some embodiments. 
         FIG. 4  is a block diagram showing an interconnect switch coupling multiple storage nodes in accordance with some embodiments. 
         FIG. 5  is a multiple level block diagram, showing contents of a storage node and contents of one of the non-volatile solid state storage units in accordance with some embodiments. 
         FIG. 6  is a flow diagram for a method of operating a storage cluster in some embodiments. 
         FIG. 7A  is a configuration diagram of data stripes of differing sizes, i.e., differing stripe widths in some embodiments. 
         FIG. 7B  is a configuration diagram of data stripes across storage nodes of various memory capacities in accordance with some embodiments. 
         FIG. 8  is a flow diagram of a method for accessing user data in storage nodes, which can be practiced on or by embodiments of the storage cluster, storage nodes and/or non-volatile solid-state storages in accordance with some embodiments. 
         FIG. 9  is an illustration showing an exemplary computing device which may implement the embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments below describe a storage cluster that stores user data, such as user data originating from one or more user or client systems or other sources external to the storage cluster. The storage cluster distributes user data across storage nodes housed within a chassis, using erasure coding and redundant copies of metadata. Erasure coding refers to a method of data protection in which data is broken into fragments, expanded and encoded with redundant data pieces and stored across a set of different locations, such as disks, storage nodes or geographic locations. Flash memory is one type of solid-state memory that may be integrated with the embodiments, although the embodiments may be extended to other types of solid-state memory or other storage medium, including non-solid state memory. Control of storage locations and workloads are distributed across the storage locations in a clustered peer-to-peer system. Tasks such as mediating communications between the various storage nodes, detecting when a storage node has become unavailable, and balancing I/Os (inputs and outputs) across the various storage nodes, are all handled on a distributed basis. Data is laid out or distributed across multiple storage nodes in data fragments or stripes that support data recovery in some embodiments. Ownership of data can be reassigned within a cluster, independent of input and output patterns. This architecture described in more detail below allows a storage node in the cluster to fail, with the system remaining operational, since the data can be reconstructed from other storage nodes and thus remain available for input and output operations. In various embodiments, a storage node may be referred to as a cluster node, a blade, or a server. 
     The storage cluster is contained within a chassis, i.e., an enclosure housing one or more storage nodes. A mechanism to provide power to each storage node, such as a power distribution bus, and a communication mechanism, such as a communication bus that enables communication between the storage nodes are included within the chassis. The storage cluster can run as an independent system in one location according to some embodiments. In one embodiment, a chassis contains at least two instances of both the power distribution and the communication bus which may be enabled or disabled independently. The internal communication bus may be an Ethernet bus, however, other technologies such as Peripheral Component Interconnect (PCI) Express, InfiniBand, and others, are equally suitable. The chassis provides a port for an external communication bus for enabling communication between multiple chassis, directly or through a switch, and with client systems. The external communication may use a technology such as Ethernet, InfiniBand, Fibre Channel, etc. In some embodiments, the external communication bus uses different communication bus technologies for inter-chassis and client communication. If a switch is deployed within or between chassis, the switch may act as a translation between multiple protocols or technologies. When multiple chassis are connected to define a storage cluster, the storage cluster may be accessed by a client using either proprietary interfaces or standard interfaces such as network file system (NFS), common internet file system (CIFS), small computer system interface (SCSI) or hypertext transfer protocol (HTTP). Translation from the client protocol may occur at the switch, chassis external communication bus or within each storage node. 
     Each storage node may be one or more storage servers and each storage server is connected to one or more non-volatile solid state memory units, which may be referred to as storage units. One embodiment includes a single storage server in each storage node and between one to eight non-volatile solid state memory units, however this one example is not meant to be limiting. The storage server may include a processor, dynamic random access memory (DRAM) and interfaces for the internal communication bus and power distribution for each of the power buses. Inside the storage node, the interfaces and storage unit share a communication bus, e.g., PCI Express, in some embodiments. The non-volatile solid state memory units may directly access the internal communication bus interface through a storage node communication bus, or request the storage node to access the bus interface. The non-volatile solid state memory unit contains an embedded central processing unit (CPU), solid state storage controller, and a quantity of solid state mass storage, e.g., between 2-32 terabytes (TB) in some embodiments. An embedded volatile storage medium, such as DRAM, and an energy reserve apparatus are included in the non-volatile solid state memory unit. In some embodiments, the energy reserve apparatus is a capacitor, super-capacitor, or battery that enables transferring a subset of DRAM contents to a stable storage medium in the case of power loss. In some embodiments, the non-volatile solid state memory unit is constructed with a storage class memory, such as phase change or magnetoresistive random access memory (MRAM) that substitutes for DRAM and enables a reduced power hold-up apparatus. 
     One of many features of the storage nodes and non-volatile solid state storage is the ability to proactively rebuild data in a storage cluster. The storage nodes and non-volatile solid state storage can determine when a storage node or non-volatile solid state storage in the storage cluster is unreachable, independent of whether there is an attempt to read data involving that storage node or non-volatile solid state storage. The storage nodes and non-volatile solid state storage then cooperate to recover and rebuild the data in at least partially new locations. This constitutes a proactive rebuild, in that the system rebuilds data without waiting until the data is needed for a read access initiated from a client system employing the storage cluster. These and further details of the storage memory and operation thereof are discussed below. 
       FIG. 1  is a perspective view of a storage cluster  160 , with multiple storage nodes  150  and internal solid-state memory coupled to each storage node to provide network attached storage or storage area network, in accordance with some embodiments. A network attached storage, storage area network, or a storage cluster, or other storage memory, could include one or more storage clusters  160 , each having one or more storage nodes  150 , in a flexible and reconfigurable arrangement of both the physical components and the amount of storage memory provided thereby. The storage cluster  160  is designed to fit in a rack, and one or more racks can be set up and populated as desired for the storage memory. The storage cluster  160  has a chassis  138  having multiple slots  142 . It should be appreciated that chassis  138  may be referred to as a housing, enclosure, or rack unit. In one embodiment, the chassis  138  has fourteen slots  142 , although other numbers of slots are readily devised. For example, some embodiments have four slots, eight slots, sixteen slots, thirty-two slots, or other suitable number of slots. Each slot  142  can accommodate one storage node  150  in some embodiments. Chassis  138  includes flaps  148  that can be utilized to mount the chassis  138  on a rack. Fans  144  provide air circulation for cooling of the storage nodes  150  and components thereof, although other cooling components could be used, or an embodiment could be devised without cooling components. A switch fabric  146  couples storage nodes  150  within chassis  138  together and to a network for communication to the memory. In an embodiment depicted in  FIG. 1 , the slots  142  to the left of the switch fabric  146  and fans  144  are shown occupied by storage nodes  150 , while the slots  142  to the right of the switch fabric  146  and fans  144  are empty and available for insertion of storage node  150  for illustrative purposes. This configuration is one example, and one or more storage nodes  150  could occupy the slots  142  in various further arrangements. The storage node arrangements need not be sequential or adjacent in some embodiments. Storage nodes  150  are hot pluggable, meaning that a storage node  150  can be inserted into a slot  142  in the chassis  138 , or removed from a slot  142 , without stopping or powering down the system. Upon insertion or removal of storage node  150  from slot  142 , the system automatically reconfigures in order to recognize and adapt to the change. Reconfiguration, in some embodiments, includes restoring redundancy and/or rebalancing data or load. 
     Each storage node  150  can have multiple components. In the embodiment shown here, the storage node  150  includes a printed circuit board  158  populated by a CPU  156 , i.e., processor, a memory  154  coupled to the CPU  156 , and a non-volatile solid state storage  152  coupled to the CPU  156 , although other mountings and/or components could be used in further embodiments. The memory  154  has instructions which are executed by the CPU  156  and/or data operated on by the CPU  156 . As further explained below, the non-volatile solid state storage  152  includes flash or, in further embodiments, other types of solid-state memory. 
       FIG. 2  is a system diagram of an enterprise computing system  102 , which can use one or more of the storage nodes, storage clusters and/or non-volatile solid state storage of  FIG. 1  as a storage resource  108 . For example, flash storage  128  of  FIG. 2  may integrate the storage nodes, storage clusters and/or non-volatile solid state storage of  FIG. 1  in some embodiments. The enterprise computing system  102  has processing resources  104 , networking resources  106  and storage resources  108 , including flash storage  128 . A flash controller  130  and flash memory  132  are included in the flash storage  128 . In various embodiments, the flash storage  128  could include one or more storage nodes or storage clusters, with the flash controller  130  including the CPUs, and the flash memory  132  including the non-volatile solid state storage of the storage nodes. In some embodiments flash memory  132  may include different types of flash memory or the same type of flash memory. The enterprise computing system  102  illustrates an environment suitable for deployment of the flash storage  128 , although the flash storage  128  could be used in other computing systems or devices, larger or smaller, or in variations of the enterprise computing system  102 , with fewer or additional resources. The enterprise computing system  102  can be coupled to a network  140 , such as the Internet, in order to provide or make use of services. For example, the enterprise computing system  102  could provide cloud services, physical computing resources, or virtual computing services. 
     In the enterprise computing system  102 , various resources are arranged and managed by various controllers. A processing controller  110  manages the processing resources  104 , which include processors  116  and random-access memory (RAM)  118 . Networking controller  112  manages the networking resources  106 , which include routers  120 , switches  122 , and servers  124 . A storage controller  114  manages storage resources  108 , which include hard drives  126  and flash storage  128 . Other types of processing resources, networking resources, and storage resources could be included with the embodiments. In some embodiments, the flash storage  128  completely replaces the hard drives  126 . The enterprise computing system  102  can provide or allocate the various resources as physical computing resources, or in variations, as virtual computing resources supported by physical computing resources. For example, the various resources could be implemented using one or more servers executing software. Files or data objects, or other forms of data, are stored in the storage resources  108 . 
     In various embodiments, an enterprise computing system  102  could include multiple racks populated by storage clusters, and these could be located in a single physical location such as in a cluster or a server farm. In other embodiments the multiple racks could be located at multiple physical locations such as in various cities, states or countries, connected by a network. Each of the racks, each of the storage clusters, each of the storage nodes, and each of the non-volatile solid state storage could be individually configured with a respective amount of storage space, which is then reconfigurable independently of the others. Storage capacity can thus be flexibly added, upgraded, subtracted, recovered and/or reconfigured at each of the non-volatile solid state storages. As mentioned previously, each storage node could implement one or more servers in some embodiments. 
       FIG. 3  is a block diagram showing multiple storage nodes  150  and non-volatile solid state storage  152  with differing capacities, suitable for use in the chassis of  FIG. 1 . Each storage node  150  can have one or more units of non-volatile solid state storage  152 . Each non-volatile solid state storage  152  may include differing capacity from other non-volatile solid state storage  152  on a storage node  150  or in other storage nodes  150  in some embodiments. Alternatively, all of the non-volatile solid state storages  152  on a storage node or on multiple storage nodes can have the same capacity or combinations of the same and/or differing capacities. This flexibility is illustrated in  FIG. 3 , which shows an example of one storage node  150  having mixed non-volatile solid state storage  152  of four, eight and thirty-two TB capacity, another storage node  150  having non-volatile solid state storage  152  each of thirty-two TB capacity, and still another storage node having non-volatile solid state storage  152  each of eight TB capacity. Various further combinations and capacities are readily devised in accordance with the teachings herein. In the context of clustering, e.g., clustering storage to form a storage cluster, a storage node can be or include a non-volatile solid state storage  152 . Non-volatile solid state storage  152  is a convenient clustering point as the non-volatile solid state storage  152  may include a nonvolatile random access memory (NVRAM) component, as will be further described below. 
     Referring to  FIGS. 1 and 3 , storage cluster  160  is scalable, meaning that storage capacity with non-uniform storage sizes is readily added, as described above. One or more storage nodes  150  can be plugged into or removed from each chassis and the storage cluster self-configures in some embodiments. Plug-in storage nodes  150 , whether installed in a chassis as delivered or later added, can have different sizes. For example, in one embodiment a storage node  150  can have any multiple of 4 TB, e.g., 8 TB, 12 TB, 16 TB, 32 TB, etc. In further embodiments, a storage node  150  could have any multiple of other storage amounts or capacities. Storage capacity of each storage node  150  is broadcast, and influences decisions of how to stripe the data. For maximum storage efficiency, an embodiment can self-configure as wide as possible in the stripe, subject to a predetermined requirement of continued operation with loss of up to one, or up to two, non-volatile solid state storage units  152  or storage nodes  150  within the chassis. 
       FIG. 4  is a block diagram showing a communications interconnect  170  and power distribution bus  172  coupling multiple storage nodes  150 . Referring back to  FIG. 1 , the communications interconnect  170  can be included in or implemented with the switch fabric  146  in some embodiments. Where multiple storage clusters  160  occupy a rack, the communications interconnect  170  can be included in or implemented with a top of rack switch, in some embodiments. As illustrated in  FIG. 4 , storage cluster  160  is enclosed within a single chassis  138 . External port  176  is coupled to storage nodes  150  through communications interconnect  170 , while external port  174  is coupled directly to a storage node. External power port  178  is coupled to power distribution bus  172 . Storage nodes  150  may include varying amounts and differing capacities of non-volatile solid state storage  152  as described with reference to  FIG. 3 . In addition, one or more storage nodes  150  may be a compute only storage node as illustrated in  FIG. 4 . Authorities  168  are implemented on the non-volatile solid state storages  152 , for example as lists or other data structures stored in memory. In some embodiments the authorities are stored within the non-volatile solid state storage  152  and supported by software executing on a controller or other processor of the non-volatile solid state storage  152 . In a further embodiment, authorities  168  are implemented on the storage nodes  150 , for example as lists or other data structures stored in the memory  154  and supported by software executing on the CPU  156  of the storage node  150 . Authorities  168  control how and where data is stored in the non-volatile solid state storages  152  in some embodiments. This control assists in determining which type of erasure coding scheme is applied to the data, and which storage nodes  150  have which portions of the data. Each authority  168  may be assigned to a non-volatile solid state storage  152 . Each authority may control a range of inode numbers, segment numbers, or other data identifiers which are assigned to data by a file system, by the storage nodes  150 , or by the non-volatile solid state storage  152 , in various embodiments. 
     Every piece of data, and every piece of metadata, has redundancy in the system in some embodiments. In addition, every piece of data and every piece of metadata has an owner, which may be referred to as an authority. If that authority is unreachable, for example through failure of a storage node, there is a plan of succession for how to find that data or that metadata. In various embodiments, there are redundant copies of authorities  168 . Authorities  168  have a relationship to storage nodes  150  and non-volatile solid state storage  152  in some embodiments. Each authority  168 , covering a range of data segment numbers or other identifiers of the data, may be assigned to a specific non-volatile solid state storage  152 . In some embodiments the authorities  168  for all of such ranges are distributed over the non-volatile solid state storages  152  of a storage cluster. Each storage node  150  has a network port that provides access to the non-volatile solid state storage(s)  152  of that storage node  150 . Data can be stored in a segment, which is associated with a segment number and that segment number is an indirection for a configuration of a RAID (redundant array of independent drives) stripe in some embodiments. The assignment and use of the authorities  168  thus establishes an indirection to data. Indirection may be referred to as the ability to reference data indirectly, in this case via an authority  168 , in accordance with some embodiments. A segment identifies a set of non-volatile solid state storage  152  and a local identifier into the set of non-volatile solid state storage  152  that may contain data. In some embodiments, the local identifier is an offset into the device and may be reused sequentially by multiple segments. In other embodiments the local identifier is unique for a specific segment and never reused. The offsets in the non-volatile solid state storage  152  are applied to locating data for writing to or reading from the non-volatile solid state storage  152  (in the form of a RAID stripe). Data is striped across multiple units of non-volatile solid state storage  152 , which may include or be different from the non-volatile solid state storage  152  having the authority  168  for a particular data segment. 
     If there is a change in where a particular segment of data is located, e.g., during a data move or a data reconstruction, the authority  168  for that data segment should be consulted, at that non-volatile solid state storage  152  or storage node  150  having that authority  168 . In order to locate a particular piece of data, embodiments calculate a hash value for a data segment or apply an inode number or a data segment number. The output of this operation points to a non-volatile solid state storage  152  having the authority  168  for that particular piece of data. In some embodiments there are two stages to this operation. The first stage maps an entity identifier (ID), e.g., a segment number, inode number, or directory number to an authority identifier. This mapping may include a calculation such as a hash or a bit mask. The second stage is mapping the authority identifier to a particular non-volatile solid state storage  152 , which may be done through an explicit mapping. The operation is repeatable, so that when the calculation is performed, the result of the calculation repeatably and reliably points to a particular non-volatile solid state storage  152  having that authority  168 . The operation may include the set of reachable storage nodes as input. If the set of reachable non-volatile solid state storage units changes the optimal set changes. In some embodiments, the persisted value is the current assignment (which is always true) and the calculated value is the target assignment the cluster will attempt to reconfigure towards. This calculation may be used to determine the optimal non-volatile solid state storage  152  for an authority in the presence of a set of non-volatile solid state storage  152  that are reachable and constitute the same cluster. The calculation also determines an ordered set of peer non-volatile solid state storage  152  that will also record the authority to non-volatile solid state storage mapping so that the authority may be determined even if the assigned non-volatile solid state storage is unreachable. A duplicate or substitute authority  168  may be consulted if a specific authority  168  is unavailable in some embodiments. 
     With reference to  FIGS. 1-4 , two of the many tasks of the CPU  156  on a storage node  150  are to break up write data, and reassemble read data. When the system has determined that data is to be written, the authority  168  for that data is located as above. When the segment ID for data is already determined the request to write is forwarded to the non-volatile solid state storage  152  currently determined to be the host of the authority  168  determined from the segment. The host CPU  156  of the storage node  150 , on which the non-volatile solid state storage  152  and corresponding authority  168  reside, then breaks up or shards the data and transmits the data out to various non-volatile solid state storage  152 . The transmitted data is written as a data stripe in accordance with an erasure coding scheme. In some embodiments, data is requested to be pulled, and in other embodiments, data is pushed. In reverse, when data is read, the authority  168  for the segment ID containing the data is located as described above. The host CPU  156  of the storage node  150  on which the non-volatile solid state storage  152  and corresponding authority  168  reside requests the data from the non-volatile solid state storage and corresponding storage nodes pointed to by the authority. In some embodiments the data is read from flash storage as a data stripe. The host CPU  156  of storage node  150  then reassembles the read data, correcting any errors (if present) according to the appropriate erasure coding scheme, and forwards the reassembled data to the network. In further embodiments, some or all of these tasks can be handled in the non-volatile solid state storage  152 . In some embodiments, the segment host requests the data be sent to storage node  150  by requesting pages from storage and then sending the data to the storage node making the original request. 
     In some systems, for example in UNIX-style file systems, data is handled with an index node or inode, which specifies a data structure that represents an object in a file system. The object could be a file or a directory, for example. Metadata may accompany the object, as attributes such as permission data and a creation timestamp, among other attributes. A segment number could be assigned to all or a portion of such an object in a file system. In other systems, data segments are handled with a segment number assigned elsewhere. For purposes of discussion, the unit of distribution is an entity, and an entity can be a file, a directory or a segment. That is, entities are units of data or metadata stored by a storage system. Entities are grouped into sets called authorities. Each authority has an authority owner, which is a storage node that has the exclusive right to update the entities in the authority. In other words, a storage node contains the authority, and that the authority, in turn, contains entities. 
     A segment is a logical container of data in accordance with some embodiments. A segment is an address space between medium address space and physical flash locations, i.e., the data segment number, are in this address space. Segments may also contain metadata, which enable data redundancy to be restored (rewritten to different flash locations or devices) without the involvement of higher level software. In one embodiment, an internal format of a segment contains client data and medium mappings to determine the position of that data. Each data segment is protected, e.g., from memory and other failures, by breaking the segment into a number of data and parity shards, where applicable. The data and parity shards are distributed, i.e., striped, across non-volatile solid state storage  152  coupled to the host CPUs  156  (See  FIG. 5 ) in accordance with an erasure coding scheme. Usage of the term segments refers to the container and its place in the address space of segments in some embodiments. Usage of the term stripe refers to the same set of shards as a segment and includes how the shards are distributed along with redundancy or parity information in accordance with some embodiments. 
     A series of address-space transformations takes place across an entire storage system. At the top is the directory entries (file names) which link to an inode. Modes point into medium address space, where data is logically stored. Medium addresses may be mapped through a series of indirect mediums to spread the load of large files, or implement data services like deduplication or snapshots. Medium addresses may be mapped through a series of indirect mediums to spread the load of large files, or implement data services like deduplication or snapshots. Segment addresses are then translated into physical flash locations. Physical flash locations have an address range bounded by the amount of flash in the system in accordance with some embodiments. Medium addresses and segment addresses are logical containers, and in some embodiments use a 128 bit or larger identifier so as to be practically infinite, with a likelihood of reuse calculated as longer than the expected life of the system. Addresses from logical containers are allocated in a hierarchical fashion in some embodiments. Initially, each non-volatile solid state storage  152  may be assigned a range of address space. Within this assigned range, the non-volatile solid state storage  152  is able to allocate addresses without synchronization with other non-volatile solid state storage  152 . 
     Data and metadata is stored by a set of underlying storage layouts that are optimized for varying workload patterns and storage devices. These layouts incorporate multiple redundancy schemes, compression formats and index algorithms. Some of these layouts store information about authorities and authority masters, while others store file metadata and file data. The redundancy schemes include error correction codes that tolerate corrupted bits within a single storage device (such as a NAND flash chip), erasure codes that tolerate the failure of multiple storage nodes, and replication schemes that tolerate data center or regional failures. In some embodiments, low density parity check (LDPC) code is used within a single storage unit. Reed-Solomon encoding is used within a storage cluster, and mirroring is used within a storage grid in some embodiments. Metadata may be stored using an ordered log structured index (such as a Log Structured Merge Tree), and large data may not be stored in a log structured layout. 
     In order to maintain consistency across multiple copies of an entity, the storage nodes agree implicitly on two things through calculations: (1) the authority that contains the entity, and (2) the storage node that contains the authority. The assignment of entities to authorities can be done by pseudo randomly assigning entities to authorities, by splitting entities into ranges based upon an externally produced key, or by placing a single entity into each authority. Examples of pseudorandom schemes are linear hashing and the Replication Under Scalable Hashing (RUSH) family of hashes, including Controlled Replication Under Scalable Hashing (CRUSH). In some embodiments, pseudo-random assignment is utilized only for assigning authorities to nodes because the set of nodes can change. The set of authorities cannot change so any subjective function may be applied in these embodiments. Some placement schemes automatically place authorities on storage nodes, while other placement schemes rely on an explicit mapping of authorities to storage nodes. In some embodiments, a pseudorandom scheme is utilized to map from each authority to a set of candidate authority owners. A pseudorandom data distribution function related to CRUSH may assign authorities to storage nodes and create a list of where the authorities are assigned. Each storage node has a copy of the pseudorandom data distribution function, and can arrive at the same calculation for distributing, and later finding or locating an authority. Each of the pseudorandom schemes requires the reachable set of storage nodes as input in some embodiments in order to conclude the same target nodes. Once an entity has been placed in an authority, the entity may be stored on physical devices so that no expected failure will lead to unexpected data loss. In some embodiments, rebalancing algorithms attempt to store the copies of all entities within an authority in the same layout and on the same set of machines. 
     Examples of expected failures include device failures, stolen machines, datacenter fires, and regional disasters, such as nuclear or geological events. Different failures lead to different levels of acceptable data loss. In some embodiments, a stolen storage node impacts neither the security nor the reliability of the system, while depending on system configuration, a regional event could lead to no loss of data, a few seconds or minutes of lost updates, or even complete data loss. 
     In the embodiments, the placement of data for storage redundancy is independent of the placement of authorities for data consistency. In some embodiments, storage nodes that contain authorities do not contain any persistent storage. Instead, the storage nodes are connected to non-volatile solid state storage units that do not contain authorities. The communications interconnect between storage nodes and non-volatile solid state storage units consists of multiple communication technologies and has non-uniform performance and fault tolerance characteristics. In some embodiments, as mentioned above, non-volatile solid state storage units are connected to storage nodes via PCI express, storage nodes are connected together within a single chassis using an Ethernet backplane, and chassis are connected together to form a storage cluster. Storage clusters are connected to clients using Ethernet or fiber channel in some embodiments. If multiple storage clusters are configured into a storage grid, the multiple storage clusters are connected using the Internet or other long-distance networking links, such as a “metro scale” link or private link that does not traverse the internet. 
     Authority owners have the exclusive right to modify entities, to migrate entities from one non-volatile solid state storage unit to another non-volatile solid state storage unit, and to add and remove copies of entities. This allows for maintaining the redundancy of the underlying data. When an authority owner fails, is going to be decommissioned, or is overloaded, the authority is transferred to a new storage node. Transient failures make it non-trivial to ensure that all non-faulty machines agree upon the new authority location. The ambiguity that arises due to transient failures can be achieved automatically by a consensus protocol such as Paxos, hot-warm failover schemes, via manual intervention by a remote system administrator, or by a local hardware administrator (such as by physically removing the failed machine from the cluster, or pressing a button on the failed machine). In some embodiments, a consensus protocol is used, and failover is automatic. If too many failures or replication events occur in too short a time period, the system goes into a self-preservation mode and halts replication and data movement activities until an administrator intervenes in accordance with some embodiments. 
     As authorities are transferred between storage nodes and authority owners update entities in their authorities, the system transfers messages between the storage nodes and non-volatile solid state storage units. With regard to persistent messages, messages that have different purposes are of different types. Depending on the type of the message, the system maintains different ordering and durability guarantees. As the persistent messages are being processed, the messages are temporarily stored in multiple durable and non-durable storage hardware technologies. In some embodiments, messages are stored in RAM, NVRAM and on NAND flash devices, and a variety of protocols are used in order to make efficient use of each storage medium. Latency-sensitive client requests may be persisted in replicated NVRAM, and then later NAND, while background rebalancing operations are persisted directly to NAND. 
     Persistent messages are persistently stored prior to being replicated. This allows the system to continue to serve client requests despite failures and component replacement. Although many hardware components contain unique identifiers that are visible to system administrators, manufacturer, hardware supply chain and ongoing monitoring quality control infrastructure, applications running on top of the infrastructure address virtualize addresses. These virtualized addresses do not change over the lifetime of the storage system, regardless of component failures and replacements. This allows each component of the storage system to be replaced over time without reconfiguration or disruptions of client request processing. 
     In some embodiments, the virtualized addresses are stored with sufficient redundancy. A continuous monitoring system correlates hardware and software status and the hardware identifiers. This allows detection and prediction of failures due to faulty components and manufacturing details. The monitoring system also enables the proactive transfer of authorities and entities away from impacted devices before failure occurs by removing the component from the critical path in some embodiments. 
     Referring to  FIGS. 1-4 , in addition to component redundancy in the communication channel, storage cluster  160  is configured to allow for the loss of one or more storage nodes  150 . In some embodiments this cluster redundancy level may be one for relatively small storage clusters  160  (less than 8 storage nodes  150 ) and two for relatively larger storage clusters  160  (8 or more storage nodes  150 ) although any number would be suitable for the cluster redundancy level. In some embodiments, where more storage nodes  150  than the redundancy level are lost, the storage cluster  160  cannot guarantee availability of data or integrity of future updates. As mentioned above, data redundancy is implemented via segments. A segment is formed by selecting equal sized shards from a subset of the non-volatile solid state storage  152 , each within a different storage node  150 . Shards are reserved to establish the redundancy level, e.g., one or two, and then a remainder constitutes the data (the data shards). The shards are encoded using an ECC scheme such as parity or Reed-Soloman (RAID 6), so that any subset of the shards equal in count to the data shards may be used to reconstruct the complete data. The storage cluster redundancy represents a minimum level of redundancy and it may be exceeded for any individual data element. Segments are stored as a set of non-volatile solid state storage units, roles (data position or parity) and allocation unit local to each non-volatile solid state storage unit. The allocation units may be a physical address or an indirection determined within the non-volatile solid state storage  152 . Each shard may be portioned into pages and each page into code words. In some embodiments, the pages are between about 4 kilobytes (kB) and 64 kB, e.g., 16 kB, while the code words are between about 512 bytes to 4 kB, e.g., 1 kB. These sizes are one example and not meant to be limiting as any suitable size for the code words and the pages may be utilized. The code words contain local error correction and a checksum to verify the error correction was successful. This checksum is “salted” with the logical address of the contents meaning that a failure to match the checksum may occur if the data is uncorrectable or misplaced. In some embodiments, when a code word fails a checksum it is converted to an “erasure” for purpose of the error correction algorithm so that the code word may be rebuilt. 
     If storage nodes  150  are added to the storage cluster  160 , the target segment width (data shards and parity shards) changes. Newly allocated segments can adopt these parameters immediately. If the new cluster configuration is too narrow to allow segments to be rebuilt to the target redundancy the segment is replaced with a new segment. If storage cluster  160  has increased the target redundancy an extra redundant shard can be allocated and generated without changing the segment contents. All other segments may remain in place without modification, leaving the system with multiple concurrent segment dimensions. When the target segment is getting wider and more efficient this information can be combined with other information to determine if the segment is a better candidate than most for background processes like wear level or garbage collection. Storage nodes  150  can have differing non-volatile solid state storage  152  of differing sizes in some embodiments. If there are many units of non-volatile solid state storage  152  of each size then the normal allocation rules for segments may apply and the larger size units of non-volatile solid state storage  152  will have more overlapping segments. The storage cluster  160  may also decide to ignore the excess space or allocate segments of narrower width to make use of the extra space. 
     Larger storage clusters  160  may be divided into storage node groups to increase the redundancy without increasing segment width. As an example, a system of 28 storage nodes  150  may be divided into two groups of 14 each with a segment size of 10+2. When a segment is allocated the system is configured to not select shards from storage nodes in multiple groups. This arrangement ensures that up to four storage nodes may be lost, i.e., two from each group and the system operates normally. Storage node groups may be aligned with a chassis to take advantage of a higher bandwidth communications interconnect  170  for redundant operations. Storage node groups can be combined without any segment reconfiguration. If a storage node group is partitioned, any segment that crosses the two storage node is partitioned. These partitioned segments which are allocated in two storage node groups have their shards realigned within storage nodes before the storage node groups are considered independent. The system may wait for shards to be rewritten according to a normal schedule, rebuild shards into the same storage node group or move the contents into another segment. 
     The total set of non-volatile solid state storage units that have at least one shard allocated to a segment may be referred to as the referenced set. These non-volatile solid state storage units (via their storage nodes) are organized first into storage node groups, with a redundancy level within each storage node group. Any subset that fulfills the redundancy level in every storage node group is a quorum set, where the entire storage cluster may operate normally. The current set of non-volatile solid state storage units that are operating and may be accessed directly or indirectly via the communication buses is referred to as the reachable set. The storage cluster may be considered to be operating when the reachable set forms a valid quorum. Devices that are referenced but not reachable may be referred to as phantom devices. All data that is stored on the device may be reconstructed from other shards in the same segment. The storage cluster attempts to dereference any phantom device by persistently storing the rebuilt shards on a reachable device. Any non-volatile solid state storage unit that is not already within a storage node that contains a segment shard is a candidate for rebuilding. In some embodiments, rebuilding proceeds by ordinary data maintenance that would lead to deallocation of the overlapping segment, similar to garbage collection. Heuristics that determine the relative efficiency in terms of reading, writing or computation may be used to decide which path is appropriate. When no segments map shards onto a particular non-volatile solid state storage unit the non-volatile solid state storage unit is considered evicted and is no longer required to participate in quorum. When there are no longer any phantom non-volatile solid state storage units, the storage cluster is at full redundancy and may lose further storage nodes while remaining in quorum. 
     In NVRAM, redundancy is not organized by segments but instead by messages, where each message (128 bytes to 128 kB) establishes its own data stripe. NVRAM is maintained at the same redundancy as segment storage and operates within the same storage node groups in some embodiments. Because messages are stored individually the stripe width is determined both by message size and the storage cluster configuration. Larger messages may be more efficiently stored as wider strips. 
       FIG. 5  is a multiple level block diagram, showing contents of a storage node  150  and contents of a non-volatile solid state storage  152  of the storage node  150 . Data is communicated to and from the storage node  150  by a network interface controller (NIC)  202  in some embodiments. Each storage node  150  has a CPU  156 , and one or more non-volatile solid state storage  152 , as discussed above. Moving down one level in  FIG. 5 , each non-volatile solid state storage  152  has a relatively fast non-volatile solid state memory, such as nonvolatile random access memory (NVRAM)  204 , and flash memory  206 . In some embodiments, NVRAM  204  may be a component that does not require program/erase cycles (DRAM, MRAM, PCM), and can be a memory that can support being written vastly more often than the memory is read from. Moving down another level in  FIG. 5 , the NVRAM  204  is implemented in one embodiment as high speed volatile memory, such as dynamic random access memory (DRAM)  216 , backed up by energy reserve  218 . Energy reserve  218  provides sufficient electrical power to keep the DRAM  216  powered long enough for contents to be transferred to the flash memory  206  in the event of power failure. In some embodiments, energy reserve  218  is a capacitor, super-capacitor, battery, or other device, that supplies a suitable supply of energy sufficient to enable the transfer of the contents of DRAM  216  to a stable storage medium in the case of power loss. The flash memory  206  is implemented as multiple flash dies  222 , which may be referred to as packages of flash dies  222  or an array of flash dies  222 . It should be appreciated that the flash dies  222  could be packaged in any number of ways, with a single die per package, multiple dies per package (i.e. multichip packages), in hybrid packages, as bare dies on a printed circuit board or other substrate, as encapsulated dies, etc. In the embodiment shown, the non-volatile solid state storage  152  has a controller  212  or other processor, and an input output (I/O) port  210  coupled to the controller  212 . I/O port  210  is coupled to the CPU  156  and/or the network interface controller  202  of the flash storage node  150 . Flash input output (I/O) port  220  is coupled to the flash dies  222 , and a direct memory access unit (DMA)  214  is coupled to the controller  212 , the DRAM  216  and the flash dies  222 . In the embodiment shown, the I/O port  210 , controller  212 , DMA unit  214  and flash I/O port  220  are implemented on a programmable logic device (PLD)  208 , e.g., a field programmable gate array (FPGA). In this embodiment, each flash die  222  has pages, organized as sixteen kB (kilobyte) pages  224 , and a register  226  through which data can be written to or read from the flash die  222 . In further embodiments, other types of solid-state memory are used in place of, or in addition to flash memory illustrated within flash die  222 . 
       FIG. 6  is a flow diagram for a method of operating a storage cluster. The method can be practiced on or by various embodiments of a storage cluster and storage nodes as described herein. Various steps of the method can be performed by a processor, such as a processor in a storage cluster or a processor in a storage node. Portions or all of the method can be implemented in software, hardware, firmware or combinations thereof. The method initiates with action  602 , where user data is distributed with erasure coding. For example, the user data could be distributed across the storage nodes of a storage cluster using one or more erasure coding schemes. Two erasure coding schemes (or more, in some embodiments) can coexist in the storage nodes in the storage cluster. In some embodiments, each storage node can determine which of a plurality of erasure coding schemes to apply when writing data, and can determine which erasure coding scheme to apply when reading data. These can be the same or differing erasure coding schemes. 
     The method proceeds to action  604  where the storage nodes in the storage cluster are checked. In some embodiments the storage nodes are checked for a heartbeat where each storage node periodically issues a message which acts as the heartbeat. In alternative embodiments the checking is in the form of querying a storage node, and the lack of response to the query indicates that the storage node is failing. In a decision action  606 , it is determined if two storage nodes are unreachable. For example, if two of the storage nodes are no longer issuing heartbeats, or two of the storage nodes fail to respond to queries, or some combination of these or other indications, one of the other storage nodes could determine that two of the storage nodes are unreachable. If this is not the situation, flow branches back to the action  602 , in order to continue distributing user data, e.g., writing user data into the storage nodes as the user data arrives for storage. If it is determined that two of the storage nodes are unreachable, flow continues to action  608 . 
     In decision action  608 , the user data is accessed in the remainder of the storage nodes, using erasure coding. It should be appreciated that user data refers to data originating from one or more users or client systems or other sources external to the storage cluster in some embodiments. In some embodiments erasure coding types include double redundancy, in which case, with two failed storage nodes, a remaining storage node has readable user data. Erasure coding types could include error correcting code allowing loss of two bits out of a code word, with data distributed across storage nodes so that the data can be recovered despite loss of two of the storage nodes. In a decision action  610 , it is determined if the data is to be rebuilt. If the data should not be rebuilt, flow branches back to the action  602 , in order to continue distributing user data with erasure coding. If the data should be rebuilt, flow branches to the action  612 . In some embodiments, the decision to rebuild the data occurs after two storage nodes are unreachable, although the decision to rebuild the data may occur after one storage node is unreachable in other embodiments. Various mechanisms that could be taken into account in the decision to rebuild the data include error correction counts, error correction rates, failures of reads, failures of writes, loss of heartbeat, failure to reply to queries, and so on. Appropriate modification to the method of  FIG. 6  is readily understood for these and further embodiments. 
     In the action  612 , the data is recovered using erasure coding. This could be according to examples of erasure coding as discussed above, regarding the action  608 . More specifically, the data is recovered from the remaining storage nodes, e.g., using error correcting code or reading from a remaining storage node, as appropriate. Data could be recovered using two or more types of erasure coding, in cases where these two or more types of erasure coding coexist in the storage nodes. In a decision action  614 , it is determined if the data should be read in parallel. In some embodiments, there is more than one data path (e.g., as with double redundancy of data), and the data could be read in parallel across two paths. If the data is not to be read in parallel, flow branches to the action  618 . If the data is to be read in parallel, flow branches to the action  616 , in which the results are raced. The winner of the race is then used as the recovered data. 
     In an action  618 , the erasure coding scheme for rebuilding is determined. For example, in some embodiments each storage node can decide which of two or more erasure coding schemes to apply when writing the data across the storage units. In some embodiments, the storage nodes cooperate to determine the erasure coding scheme. This can be done by determining which storage node has responsibility for the erasure coding scheme for a specific data segment, or by assigning a storage node to have this responsibility. In some embodiments, various mechanisms such as witnessing, voting, or decision logic, and so on, are employed to achieve this action. Non-volatile solid state storage may act as witnesses (in some embodiments) or voters (in some embodiments), so that if one copy of an authority becomes defective, the remaining functioning non-volatile solid state storage and the remaining copies of the authorities can determine the contents of the defective authority. In an action  620 , the recovered data is written across the remainder of the storage nodes, with erasure coding. For example, the erasure coding scheme that is determined for the rebuilding could be different from the erasure coding scheme that is applied in recovering the data, i.e., in reading the data. More specifically, loss of two of the storage nodes may mean that some erasure coding schemes are no longer applicable to the remaining storage nodes, and the storage nodes then switch to an erasure coding scheme that is applicable to the remaining storage nodes. 
     The ability to change configuration of data stripes  314  is one of many features of storage nodes  150  and non-volatile solid-state storages  152 . In the examples of  FIGS. 7A and 7B  data is stored in the form of data stripes  314 ,  326 ,  338 ,  340 ,  342 ,  344 , in which data is sharded, i.e., broken up and distributed, across multiple storage nodes  150 . In some embodiments, data can be striped across non-volatile solid-state storages  152  in a storage node  150  or in multiple storage nodes  150 . Various RAID configurations and associated data striping schemes and levels of redundancy are possible, as controlled by the authority  168  for each data segment. A storage node  150  having an authority  168  for a specified data segment could be a storage node  150  acting as a data server, a storage node  150  acting as a parity server, or a storage node  150  having no solid-state storage  152  (compute only). In various arrangements, each storage node  150 , or each non-volatile solid-state storage  152 , provides one bit of data or one parity bit, for a data stripe  314 . Various embodiments implement an error correction code (ECC) that allows recovery of data even if one or two storage nodes  150  fail or are unavailable. In some embodiments, the storage node holding the authority  168  determines which RAID scheme or level and that authority  168  so points to the data stripe  314 . Data striping can be applied at a bit level, byte level or block level, and further data striping schemes are possible. In some versions, a storage node  150 , or a solid-state storage  152 , could contribute more than one bit to a data stripe  314 . In some embodiments, an error correction code calculation is performed at each non-volatile solid-state storage  152 , for the shard of data that the non-volatile solid-state storage  152  contains from a segment. That shard of data, corrected as needed, is sent back to the storage node  150  that has the authority  168  for the data, where the data is reassembled. More than one stripe type, or RAID scheme or level, can be present across the storage nodes  150  (i.e., coexist in the storage cluster  160 ), as will be further described below. 
       FIG. 7A  is a configuration diagram of data stripes  314 ,  326  of differing sizes, i.e., differing stripe widths. These data stripes  314 ,  326  coexist across the storage nodes  150 , or in some embodiments coexist across non-volatile solid-state storage  152 . For example, one data stripe  326  is sharded across the three storage nodes  150  in a RAID scheme or level having double redundancy. Identical copies of the data from a first storage node  150  are present in each of the second storage node  150  and the third storage node  150 . Since this version of data recovery, in the example data stripe  326 , requires two identical copies of data, the storage overhead is 200% more than the data storage capacity. In other words, the relative total storage amount is (N+2N) divided by N, which equals three, for N bits of data, and this is independent of N. Another data stripe  314  is sharded across storage nodes  150  acting as data servers and across storage nodes  150  acting as parity servers providing the p and q parity bits. User data is written to and read from the storage nodes  150  in accordance with a RAID scheme or level having two parity bits, i.e., parity bit p and parity bit q, in this data stripe  314 . Since this particular error correction code adds two bits to the data length, the relative storage overhead is related to (N+2) divided by N, for N bits of data. For example, 10 plus 2 redundancy has 20% overhead of memory. The wider data stripe  314  therefore has greater storage efficiency and lower storage overhead than the narrower data stripe  326 . 
       FIG. 7B  is a configuration diagram of data stripes  338 ,  340 ,  342 ,  344  across storage nodes  150  of various memory capacities in accordance with some embodiments. As illustrated, two of the storage nodes  150  have greater capacity than two others of the storage nodes  150 , for example by a factor of two. All of the capacity of these storage nodes  150  can be used by applying data stripes  338 ,  340 ,  342 ,  344  as shown. For example, two data stripes  338 ,  340  are applied across two of the higher capacity storage nodes  150  and one of the lower capacity storage nodes  150 . Two more data stripes  342 ,  344  are applied across one of the lower capacity storage nodes  150  and two of the higher capacity storage nodes  150 . 
     The ability of various embodiments to self-configure, on power up or upon removal, replacement or insertion of one or more storage nodes  150  and/or solid-state storages  152  provides a storage memory topology that automatically reconfigures. For example, in a storage cluster with multiple storage nodes  150  and two levels of redundancy, two storage nodes  150  could be lost and data could still be reconstructed, as described above. In a small configuration, the storage cluster  160  could self-configure to store two replicated copies, i.e., mirrors of the data, with 200% storage overhead. In a larger configuration, the cluster could self-configure to have parity pages, with a lower storage overhead. Storage overhead is thus reconfigured as cluster membership changes. The storage nodes  150 , solid-state storages  152 , and storage cluster  160  which these form can dynamically switch between RAID schemes, and at any moment could have a hybrid combination of RAID schemes. Earlier-formed stripes do not need to be reconstructed when the topology of the storage cluster  160  changes and can be left as is, or reconstructed later according to a new RAID scheme. Storage nodes  150 , and non-volatile solid-state storage  152  can switch from one data striping scheme to another in subsequent accesses, i.e., writes or reads, in some embodiments. New data that is arriving can be written to whichever topology is in place at the moment of the data arrival. Adding one or more storage nodes  150 , or non-volatile solid-state storage  152 , does not require that data be removed from the system for system repartitioning. The topology of the storage cluster  160 , e.g., the RAID scheme(s) and storage overhead, are automatically reconfigured as the geometry of the storage cluster  160  and/or storage capacity of each storage node  150  or each non-volatile solid-state storage  152 , is changed. The storage nodes  150  and non-volatile solid-state storage  152  thus implement dynamically switching between data striping, e.g., RAID, schemes in a hybrid topology of storage memory. 
     This flexibility of a storage cluster  160  with storage nodes  150  with regard to data stripes of differing sizes can be exploited during upgrades, data recovery, reconstruction, redistribution, reassignment, etc. For example, a smaller system could have 2 plus 1 redundancy, meaning each data has an identical copy and there is 100% overhead of memory. A larger system could have 10 plus 2 redundancy, meaning there is only 20% overhead of memory. Adding storage capacity, with self-configuring of the system as to redundancy, thus decreases storage overhead and increases storage efficiency. Storage nodes  150  have the capability to determine data striping schemes that make use of essentially all of the available storage capacity of the storage nodes  150 . In contrast, fixed data striping schemes waste storage capacity of storage nodes  150  having greater capacity than whichever storage node  150  has the minimum storage capacity of the storage cluster  160 . A system could have some data protected by one redundancy scheme, and some data by another redundancy scheme. Data could be protected by different levels of redundancy as a system grows. Hence, numerous data protection schemes may be integrated into the embodiments depending on the application or other desired characteristics. This is a distributed role, in that each of the storage nodes  150  has the ability to independently select the RAID stripe scheme. The storage cluster  160 , on a distributed basis, decides where to place the data as the data arrives for writing in some embodiments. 
     Embodiments as described above provide for a non-disruptive upgrade. This compares to former methods of upgrading, including one method known colloquially as a “forklift upgrade”, in which the data must be migrated off of components that are being replaced. Components are then removed and replaced with upgraded components, and the data is migrated back into the new components. In the presently described storage cluster  160 , components can be replaced or added, and the system remains online and accessible during the upgrade process. The storage cluster  160  reconfigures to absorb the new components. A full upgrade of the system may occur incrementally through the embodiments described herein. For example, as newer solid-state technology is developed, where the technology has different size limitations than previous generations, the embodiments enable the introduction of the newer solid-state technology to replace a defective storage node, add additional capacity, take advantage of newer technology, etc. In time, the entire system may be replaced and/or upgraded through the incremental storage node replacements. In addition to adding capacity, the embodiments also cover the deletion of capacity in a non-disruptive manner. 
       FIG. 8  is a flow diagram of a method for accessing user data in storage nodes, which can be practiced on or by embodiments of the storage cluster, storage nodes and/or non-volatile solid-state storages in accordance with some embodiments. Many of the actions described in the method can be performed by one or more processors, such as processors on storage nodes and processors in solid-state storages. Portions or all of the method can be implemented in software, hardware, firmware or combinations thereof. The storage nodes and solid-state storages can have same or differing capacities as illustrated in  FIGS. 3, 6 and 7 . In the method, the storage nodes and solid-state storages can reconfigure if a storage node is added or removed, and can access and reconstruct user data even if two storage nodes fail, all while supporting non-uniform storage sizes and scalability. 
     In an action  802 , user data is distributed throughout the storage nodes of a storage cluster. For example, the storage nodes can self-configure as to one or more data striping schemes that shard data across the storage nodes. Each data striping scheme is in accordance with a level of redundancy of user data. The user data could be distributed across the storage nodes of a storage cluster using one or more erasure coding schemes. Two erasure coding schemes (or more, in some embodiments) can coexist in the storage nodes in the storage cluster. In some embodiments, each storage node can determine which of a plurality of erasure coding schemes to apply when writing data, and can determine which erasure coding scheme to apply when reading data. These can be the same or differing erasure coding schemes. 
     In a decision action  804 , it is determined if two storage nodes are unreachable. If the answer is no, flow branches back to the action  802 , in order to continue distributing user data throughout the storage nodes. If the answer is yes, flow branches to the action  806 . In the action  806 , the user data is accessed from the remaining storage nodes, via the erasure coding. This access could include reading or writing the user data, or reconstructing the user data. For example, since two storage nodes have failed, the user data could be accessed in a redundant copy of the user data in one data striping scheme (e.g., RAID 1), or error correction could be applied to read data from storage nodes applying a data striping scheme (e.g., RAID 6) using N plus 2 redundancy. 
     In an action  808 , a storage node is added. The storage node could have the same storage capacity as other storage nodes, or a differing storage capacity. In an action  810 , an erasure coding scheme is determined. For example, adding the storage node could result in the storage nodes determining that a particular erasure coding scheme is suitable for the new total number of storage nodes and makes use of all of the storage capacity of the storage nodes. In an action  812 , the storage nodes are configured. This configuration is according to the erasure coding scheme determined in the action  810 . In some embodiments the configuration in action  812  initiates automatically in response to detection of the storage node being added. 
     In an action  814 , the user data is recovered. For example, the user data could be recovered from the remaining storage nodes, similar to the access in the action  806 . The storage nodes could apply the appropriate erasure coding scheme to recover the user data. In an action  816 , the user data is written. For example, the user data that was recovered in the action  814  could be written to the storage nodes configured in the action  812 , which is according to the erasure coding scheme determined in the action  810 . 
     It should be appreciated that the methods described herein may be performed with a digital processing system, such as a conventional, general-purpose computer system. Special purpose computers, which are designed or programmed to perform only one function may be used in the alternative.  FIG. 9  is an illustration showing an exemplary computing device which may implement the embodiments described herein. The computing device of  FIG. 9  may be used to perform embodiments of the functionality for a storage node or a non-volatile solid-state storage in accordance with some embodiments. The computing device includes a central processing unit (CPU)  901 , which is coupled through a bus  905  to a memory  903 , and mass storage device  907 . Mass storage device  907  represents a persistent data storage device such as a disc drive, which may be local or remote in some embodiments. The mass storage device  907  could implement a backup storage, in some embodiments. Memory  903  may include read only memory, random access memory, etc. Applications resident on the computing device may be stored on or accessed via a computer readable medium such as memory  903  or mass storage device  907  in some embodiments. Applications may also be in the form of modulated electronic signals modulated accessed via a network modem or other network interface of the computing device. It should be appreciated that CPU  901  may be embodied in a general-purpose processor, a special purpose processor, or a specially programmed logic device in some embodiments. 
     Display  911  is in communication with CPU  901 , memory  903 , and mass storage device  907 , through bus  905 . Display  911  is configured to display any visualization tools or reports associated with the system described herein. Input/output device  909  is coupled to bus  905  in order to communicate information in command selections to CPU  901 . It should be appreciated that data to and from external devices may be communicated through the input/output device  909 . CPU  901  can be defined to execute the functionality described herein to enable the functionality described with reference to  FIGS. 1-6 . The code embodying this functionality may be stored within memory  903  or mass storage device  907  for execution by a processor such as CPU  901  in some embodiments. The operating system on the computing device may be MS-WINDOWS™ UNIX™ LINUX™, iOS™, CentOS™, Android™, Redhat Linux™, z/OS™, or other known operating systems. It should be appreciated that the embodiments described herein may be integrated with virtualized computing system also. 
     Detailed illustrative embodiments are disclosed herein. However, specific functional details disclosed herein are merely representative for purposes of describing embodiments. Embodiments may, however, be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
     It should be understood that although the terms first, second, etc. may be used herein to describe various steps or calculations, these steps or calculations should not be limited by these terms. These terms are only used to distinguish one step or calculation from another. For example, a first calculation could be termed a second calculation, and, similarly, a second step could be termed a first step, without departing from the scope of this disclosure. As used herein, the term “and/or” and the “/” symbol includes any and all combinations of one or more of the associated listed items. 
     As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Therefore, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. 
     It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     With the above embodiments in mind, it should be understood that the embodiments might employ various computer-implemented operations involving data stored in computer systems. These operations are those requiring physical manipulation of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. Further, the manipulations performed are often referred to in terms, such as producing, identifying, determining, or comparing. Any of the operations described herein that form part of the embodiments are useful machine operations. The embodiments also relate to a device or an apparatus for performing these operations. The apparatus can be specially constructed for the required purpose, or the apparatus can be a general-purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general-purpose machines can be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations. 
     A module, an application, a layer, an agent or other method-operable entity could be implemented as hardware, firmware, or a processor executing software, or combinations thereof. It should be appreciated that, where a software-based embodiment is disclosed herein, the software can be embodied in a physical machine such as a controller. For example, a controller could include a first module and a second module. A controller could be configured to perform various actions, e.g., of a method, an application, a layer or an agent. 
     The embodiments can also be embodied as computer readable code on a non-transitory computer readable medium. The computer readable medium is any data storage device that can store data, which can be thereafter read by a computer system. Examples of the computer readable medium include hard drives, network attached storage (NAS), read-only memory, random-access memory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes, and other optical and non-optical data storage devices. The computer readable medium can also be distributed over a network coupled computer system so that the computer readable code is stored and executed in a distributed fashion. Embodiments described herein may be practiced with various computer system configurations including hand-held devices, tablets, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers and the like. The embodiments can also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a wire-based or wireless network. 
     Although the method operations were described in a specific order, it should be understood that other operations may be performed in between described operations, described operations may be adjusted so that they occur at slightly different times or the described operations may be distributed in a system which allows the occurrence of the processing operations at various intervals associated with the processing. 
     In various embodiments, one or more portions of the methods and mechanisms described herein may form part of a cloud-computing environment. In such embodiments, resources may be provided over the Internet as services according to one or more various models. Such models may include Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as a Service (SaaS). In IaaS, computer infrastructure is delivered as a service. In such a case, the computing equipment is generally owned and operated by the service provider. In the PaaS model, software tools and underlying equipment used by developers to develop software solutions may be provided as a service and hosted by the service provider. SaaS typically includes a service provider licensing software as a service on demand. The service provider may host the software, or may deploy the software to a customer for a given period of time. Numerous combinations of the above models are possible and are contemplated. 
     Various units, circuits, or other components may be described or claimed as “configured to” perform a task or tasks. In such contexts, the phrase “configured to” is used to connote structure by indicating that the units/circuits/components include structure (e.g., circuitry) that performs the task or tasks during operation. As such, the unit/circuit/component can be said to be configured to perform the task even when the specified unit/circuit/component is not currently operational (e.g., is not on). The units/circuits/components used with the “configured to” language include hardware—for example, circuits, memory storing program instructions executable to implement the operation, etc. Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. 112, sixth paragraph, for that unit/circuit/component. Additionally, “configured to” can include generic structure (e.g., generic circuitry) that is manipulated by software and/or firmware (e.g., an FPGA or a general-purpose processor executing software) to operate in manner that is capable of performing the task(s) at issue. “Configured to” may also include adapting a manufacturing process (e.g., a semiconductor fabrication facility) to fabricate devices (e.g., integrated circuits) that are adapted to implement or perform one or more tasks. 
     The foregoing description, for the purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the embodiments and its practical applications, to thereby enable others skilled in the art to best utilize the embodiments and various modifications as may be suited to the particular use contemplated. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.