# Patent

## Title
Masking defective bits in a storage array

## Abstract

A method of failure mapping is provided. The method includes distributing user data throughout a plurality of storage nodes through erasure coding, wherein the plurality of storage nodes are housed within a chassis that couples the storage nodes as a storage cluster. Each of the plurality of storage nodes has a non-volatile solid-state storage with flash memory or other types of non-volatile memory and the user data is accessible via the erasure coding from a remainder of the plurality of storage nodes in event of two of the plurality of storage nodes being unreachable. The method includes determining that a non-volatile memory block in the memory has a defect and generating a mask that indicates the non-volatile memory block and the defect. The method includes reading from the non-volatile memory block with application of the mask, wherein the reading and the application of the mask are performed by the non-volatile solid-state storage.

## Background
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 operation, lifespan, defect and other 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. Address spaces optimized for spinning media may be suboptimal for solid-state memory.
It is within this context that the embodiments arise.
SUMMARY
In some embodiments, a method of failure mapping is provided. The method includes distributing user data throughout a plurality of storage nodes through erasure coding, wherein the plurality of storage nodes are housed within a chassis that couples the storage nodes as a storage cluster. Each of the plurality of storage nodes has a non-volatile solid-state storage with non-volatile memory and the user data is accessible via the erasure coding from a remainder of the plurality of storage nodes in event of two of the plurality of storage nodes being unreachable. The method includes determining that a non-volatile memory block in the memory has a defect and generating a mask that indicates the flash block and the defect. The method includes reading from the non-volatile memory block with application of the mask, wherein the reading and the application of the mask are performed by the non-volatile solid-state storage.
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 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. 4 is a block diagram showing a communication path for redundant copies of metadata, with further details of storage nodes and solid-state storages in accordance with some embodiments.
FIG. 5 is an address and data diagram showing address translation as applied to user data being stored in a non-volatile solid-state storage in some embodiments.
FIG. 6 is a multiple level block diagram, showing a controller, flash dies, and interior details of flash dies.
FIG. 7 illustrates failure mapping, in which addresses are mapped around defects in flash memory, in some embodiments.
FIG. 8A illustrates a defective flash block in one of the flash dies in a flash memory.
FIG. 8B shows a mask with a defect indicator, which can be applied to mask a defect in the defective flash block of FIG. 8A.
FIG. 8C shows various strategies and mechanisms for applying the mask shown in FIG. 8B to reads of data in the flash block of FIG. 8A.
FIG. 9 is a flow diagram of a method for masking defective bits in a storage array, which can be practiced on or by the storage cluster, storage nodes and/or non-volatile solid-state storages in accordance with some embodiments.
FIG. 10 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 non-volatile solid-state storages or 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 non-volatile solid-state storage 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.
The storage nodes have one or more non-volatile solid-state storage units, each of which has non-volatile random-access memory (NVRAM) and flash memory, in some embodiments. The non-volatile solid-state storage units apply various address spaces for storing user data. The address spaces, and assignments of addresses to data segments and data shards, may be tracked in mapping tables, which are implemented as metadata in various locations in memory. In some embodiments, an address space has sequential, nonrepeating addresses, as applied to medium addresses, segment addresses and/or virtual allocation units of the user data. In various embodiments, the address space can be ever-increasing, ever-decreasing or some other nonrepeating sequence of values. For simplicity, the ever-increasing, nonrepeating addresses may be used as one example in the embodiments but is not meant to be limiting. This mechanism enhances the ability to write to pages in flash memory, and for reading the flash memory to recover a previous version of user data. In a storage cluster, the non-volatile solid-state storage units are assigned non-overlapping ranges from this address space.
One of many features of the storage nodes and non-volatile solid-state storage units described below is the ability to mask defective bits when reading from the flash memory. The non-volatile solid-state storage units generate masks based on defects in the flash memory. There are various mechanisms to apply a mask, and various embodiments may make use of one or more related mechanisms. Bit values can be replaced, bit values can be removed and substitute data inserted, or bit locations can be masked so as to indicate to error correction a don't care, or a probability of a stuck bit in some embodiments. Allowing continued use of flash dies with defective blocks or pages, and flash packages with defective flash dies, supports yield recovery, use of all available storage space, and virtualizing the capacity of the system. Die packages that could not ordinarily be sold in the marketplace can be used herein. The detection of defects, and application of masking, can be performed dynamically, which supports graceful degradation of storage capacity without catastrophic failure.
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 single 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.
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. 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 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. 3, each non-volatile solid-state storage 152 has a relatively fast non-volatile solid-state memory, such as non-volatile 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. 3, 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 dies on a printed circuit board or other substrate. In some embodiments, the hybrid package may include a combination of memory types, such as NVRAM, random access memory (RAM), CPU, field programmable gate array (FPGA), or different sized flash memory in the same package. 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.
In NVRAM 204, redundancy is not organized by segments but instead by messages, where each message (e.g., 128 bytes to 128 kB or smaller or larger) establishes its own data stripe, in some embodiments. 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.
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, an authority for that data is located in one of the non-volatile solid-state storages 152. The authority, i.e., the owner of the metadata or user data, may be embodied as metadata, including one or more lists such as lists of data segments which the non-volatile solid-state storage 152 manages. When a 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 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 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 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 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 embodiments, a stripe width is only read if there is a single page grid failure or delay.
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 may be an address space between medium address space and physical flash locations. 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, ie., striped, across non-volatile solid-state storages 152 coupled to the host CPUs 156 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 are the directory entries (filenames), which link to an inode ID. 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. 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 are 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. Data is not further replicated within a storage cluster, as it is assumed a storage cluster may fail. 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.
FIG. 4 is a block diagram showing a communication path 234 for redundant copies of metadata 230, with further details of flash storage nodes 150 (i.e., storage nodes 150 having flash memory) and non-volatile solid-state storages 152 in accordance with some embodiments. Metadata 230 includes information about the user data that is written to or read from the flash memory 206. Metadata 230 can include messages, or derivations from the messages, indicating actions to be taken or actions that have taken place involving the data that is written to or read from the flash memory 206. Distributing redundant copies of metadata 230 to the non-volatile solid-state storage units 152 through the communication interconnect 170 ensures that messages are persisted and can survive various types of failure the system may experience. Each non-volatile solid-state storage 152 dedicates a portion of the NVRAM 204 to storing metadata 230. In some embodiments, redundant copies of metadata 230 are stored in the additional non-volatile solid-state storage 152.
Flash storage nodes 150 are coupled via the communication interconnect 170. More specifically, the network interface controller 202 of each storage node 150 in the storage cluster is coupled to the communication interconnect 170, providing a communication path 234 among storage nodes 150 and non-volatile solid-state storage 152. Storage nodes 150 have one or more non-volatile solid-state storage units 152. Non-volatile solid-state storage units 152 internal to a storage node can communicate with each other, for example via a bus, a serial communication path, a network path or other communication path 234 as readily devised in accordance with the embodiments disclosed herein. Communication interconnect 170 can be included in or implemented with the switch fabric of FIG. 1 in some embodiments. Storage nodes 150 of FIG. 4 form a storage cluster that is enclosed within a single chassis that has an internal power distribution bus within the chassis as described with reference to FIG. 1.
Referring to FIGS. 3 and 4, in case of a power failure, whether local to non-volatile solid-state storage 152 or a storage node 150, data can be copied from the NVRAM 204 to the flash memory 206. For example, the DMA unit 214 of FIG. 3 can copy contents of the NVRAM 204, including the metadata, to the flash memory 206, using power supplied by the energy reserve 218. Energy reserve 218 is sized with sufficient capacity to support copy operation. That is, the energy reserve 218 should be sized so as to provide sufficient current at a sufficient voltage level for a time duration long enough to complete the copying so that messages that are in metadata 230 are persisted in the flash memory 206.
A further mechanism for persisting messages in a storage system involves the communication path 234 described above in FIG. 4. Redundant copies of the metadata 230 can be distributed via the communication path 234, in various ways. For example, a message coming from the filesystem could be distributed via the communication interconnect 170 as a broadcast over the communication path 234 to all of the non-volatile solid-state storages 152. A non-volatile solid-state storage 152 could send a copy of metadata 230 over the communication path 234 to other non-volatile solid-state storage 152 in a storage node 150. CPU 156 on a storage node 150, receiving a message from the communication interconnect 170 via the network interface controller 202 could send a copy of the message to each solid-state storage 152. The CPU 156 may rebroadcast the message to other flash storage nodes 150, and the flash storage nodes 150 could then distribute the message to the solid-state storages 152 in each of these flash storage nodes 150 in some embodiments. In these and other uses of the communication path 234, redundant copies of the metadata 230 can be distributed to the non-volatile solid-state storages 152. Then, if one non-volatile solid-state storage 152, or one storage node 150 experiences a failure, redundant copies of any message are available in metadata 230 of at least one other non-volatile solid-state storage 152. Each non-volatile solid-state storage 152 can apply decision logic 232 when evaluating various situations such as local power failure, an unreachable node, or instructions to consider or commence a data recovery or a data rebuild. The decision logic 232 includes witnessing logic, voting logic, consensus logic and/or other types of decision logic in various embodiments. Decision logic 232 could be implemented in hardware, software executing on the controller 212, firmware, or combinations thereof, and could be implemented as part of the controller 212 or coupled to the controller 212. The decision logic 232 is employed in consensus decisions among multiple solid-state storage units 152, in some embodiments. In further embodiments, the decision logic 232 could cooperate with the other non-volatile solid-state storage units 152 in order to gather copies of the redundant metadata 230, and make local decisions. The mechanisms for persisting messages in a storage system are useful in the event of a failure, and can be used in data recovery and reconstruction as described above.
Examples of messages include a request to write data, a request to read data, a request to lock or unlock a file, a change in permission of a file, an update to a file allocation table or other file or directory structure, a request to write a file that has executable instructions or to write a file name that is reserved and interpreted as an executable direction, updates to one or more authorities, updates to a fingerprint table, list or other data used in deduplication, updates to hash tables, updates to logs, and so on. When a message is received in non-volatile solid-state storage 152 of a storage node 150, indicating some action has taken place, the message or a derivation of the message is stored as metadata 230 in the NVRAM 204 of that solid-state storage 152. By applying the redundant copies of the metadata 230, actions are captured that are in progress, so that if a failure happens, these actions can be replayed and replacement actions can then be performed, for example upon restart. Actions span storage nodes and use cluster messaging, so the act of sending a message can be made persistent data via one or more of the mechanisms for persisting messages. These mechanisms address some of the known failure scenarios in order to ensure availability of data. In some embodiments, the messages don't require permanence beyond completion of the actions. In other embodiments the messages are further retained to facilitate rollback or other recovery operations.
For example, if a command is sent out to carry out a write operation, this message is recorded and redundant. If there is a failure, it can be determined whether or not that action has been carried out, and whether or not the action should be driven to completion. Such determination can be carried out using the decision logic 232 in each non-volatile solid-state storage 152. There is dedicated storage in NVRAM 204 for messages and other metadata 230, so that messages are recorded in the non-volatile solid-state storage 152 and replicated in some embodiments. The messages and other metadata 230 are written into flash memory 206 if one non-volatile solid-state storage 152 experiences a power failure, or if the entire system experiences a power failure or otherwise shuts down. The redundancy level of the messages matches the redundancy level of the metadata in some embodiments. When there are sufficient numbers of copies of messages, the message becomes irrevocable. If one node goes down, other nodes can vote, achieve consensus, or witness the various copies of the message and determine what action, if any, to carry to completion. If the entire system goes down, e.g., through a global power failure, then a sufficient number of these messages get written from NVRAM 204 to flash memory 206. Upon restoration of power, the nodes can again open copies of the message and determine what action, if any, to carry to completion to prevent any corruption.
With continued reference to FIGS. 3 and 4, storage node 150 of a storage cluster 160 includes two levels of controllers. There is a host CPU 156 in the storage node 150, and there is a controller 212 in the non-volatile solid-state storage 152. The controller 212 can be considered a flash memory controller, which serves as a bridge between the host CPU 156 and the flash memory 206. Each of these controllers, namely the host CPU 156 and the flash controller 212, can be implemented as one or more processors or controllers of various types from various manufacturers. The host CPU 156 can access both the flash memory 206 and the NVRAM 204 as distinct resources, with each being independently (i.e., individually) addressable by the host CPU 156.
By separating the NVRAM 204 and the flash memory 206 into distinct resources, not all data placed in the NVRAM 204 must be written to the flash memory 206. The NVRAM 204 can also be employed for various functions and purposes. For example, updates to the NVRAM 204 can be made obsolete by newer updates to the NVRAM 204. A later transfer of user data from the NVRAM 204 to the flash memory 206 can transfer the updated user data, without transferring the obsolete user data to the flash memory 206. This reduces the number of erasure cycles of the flash memory 206, reduces wear on the flash memory 206, and moves data more efficiently. The CPU 156 can write to the NVRAM 204 at a smaller granularity than the granularity of the transfers from the NVRAM 204 to the flash memory 206. For example, the CPU 156 could perform 4 kB writes to the NVRAM 204, and the DMA unit 214 could perform a page write of 16 kB from the NVRAM 204 to the flash memory 206 under direction of the controller 212. The ability to collect multiple writes of user data to the NVRAM 204 prior to writing the user data from the NVRAM 204 to the flash memory 206 increases writing efficiency. In some embodiments, a client write of user data is acknowledged at the point at which the user data is written to the NVRAM 204. Since the energy reserve 218, described above with reference to FIG. 3, provides sufficient power for a transfer of contents of the NVRAM 204 to the flash memory 206, the acknowledgment of the client write does not need to wait until the user data is written to the flash memory 206.
As further examples of differences between present embodiments and previous solid-state drives, the metadata 230 in the NVRAM 204 is not written into the flash memory 206, except in cases of power loss. Here, a portion of the NVRAM 204 acts as a workspace for the CPU 156 of the storage node 150 to apply the metadata 230. The CPU 156 of the storage node 150 can write to the NVRAM 204 and read the NVRAM 204, in order to access the metadata 230. The CPU 156 is responsible for migrating data from the NVRAM 204 down to the flash memory 206 in one embodiment. Transfer from the NVRAM 204 to the flash memory 206 is not automatic and predetermined, in such embodiments. Transfer waits until there is sufficient user data in the NVRAM 204 for a page write to the flash memory 206, as determined by the CPU 156 and directed to the DMA unit 214. The DMA unit 214 can be further involved in the path of the user data. In some embodiments, the DMA unit 214 (also known as a DMA engine) is designed to detect and understand various data formats. The DMA unit 214 can perform a cyclic redundancy check (CRC) calculation to check the integrity of the user data. In some embodiments, the DMA unit 214 inserts the CRC calculation into the data and verifies that the data is consistent with a previously inserted CRC calculation.
Work may be offloaded to the controller 212 of the non-volatile solid-state storage 152. Processing that is offloaded to flash controller 212 can be co-designed with processing performed by the CPU 156 of the storage node 150. Various mapping tables that translate from one address space to another, e.g., index trees or address translation tables, can be managed within the non-volatile solid-state storage 152, in some embodiments. The controller 212 of the non-volatile solid-state storage 152 can perform various tasks such as looking through these mapping tables, finding metadata associated with the mapping tables, and determining physical addresses, e.g., for user data sought by the CPU 156 of the storage node 150. In order to find an authority associated with a segment number, a standard solid-state drive might bring back an entire 16 kB flash page, and the CPU 156 would search in this page. In some embodiments, the controller 212 of the non-volatile solid-state storage 152 can perform this search much more efficiently, and pass the results to the CPU 156 of the storage node 150, without sending back the entire flash page to the CPU 156.
FIG. 5 is an address and data diagram showing address translation as applied to user data being stored in an embodiment of a non-volatile solid-state storage 152. In some embodiments, one or more of the address translations applies an address space having sequential, nonrepeating addresses. Addresses in this address space could be in an ever-increasing sequence (e.g., counting numbers or a regular or irregular counting sequence with skipping), an ever-decreasing sequence (e.g., a countdown or a regular or irregular countdown with skipping), a pseudorandom sequence generated from one or more specified or generated seed numbers, a Fibonacci sequence, geometric sequence or other mathematical sequence, etc. Further nonrepeating sequences are readily devised in accordance with the teachings herein. User data, arriving for storage in a storage cluster, is associated with a file path according to a file system. The user data is separated into data segments, each of which is assigned a segment address. Each data segment is separated into data shards, each of which is stored in flash memory 206. Various address translation tables 502 (e.g., mapping tables) are applied by either the CPU of the storage node or the controller of the non-volatile solid-state storage to translate, track and assign addresses to the user data and portions thereof.
These address translation tables 502 reside as metadata in the memory 154 (See FIG. 1) of the storage node, the NVRAM 204 of the non-volatile solid-state storage, and/or the flash memory of the non-volatile solid-state storage, in various embodiments. Generally, address translation tables 502 of FIG. 5 with a greater number of entries as result of being later in the chain of translations (e.g., address translation tables 502D and 502E) should be located in the flash memory 206, as there may not be sufficient memory space for these in the NVRAM or the memory 154. Further, messages regarding updates to the tables 502, or derivations of these messages, could be stored as metadata in the above-described memories. Metadata in one or more of these locations can be subjected to replication, i.e., redundancy, and decisions for various degrees of fault tolerance and system recovery, as described above.
For a particular portion of user data, the file path is translated or mapped to an inode ID with use of an address translation table 502A. This may be in accordance with a filesystem, and could be performed by the CPU of the storage node in some embodiments. The inode ID is translated or mapped to a medium address with use of an address translation table 502B, which could be performed by a CPU. In some embodiments, the medium address, which is in a medium address space, is included as one of the sequential, nonrepeating addresses. The medium address is translated or mapped to the segment address, with use of an address translation table 502C through the CPU in some embodiments. The segment address, which is in a segment address space, may be included as one of the sequential, nonrepeating addresses. The segment address, as assigned to the data segment, is translated to a virtual allocation unit, as assigned to the data shard, with use of an address translation table 502D. Controller 212 of the non-volatile solid-state storage may perform this translation by accessing address translation table 502D in the flash memory 206. The virtual allocation unit is translated to a physical flash memory location with the use of an address translation table 502E. The physical flash memory location may be assigned to the data shard in some embodiments.
The address space with the sequential, nonrepeating addresses may be applied to the medium address space, the segment address space and/or the virtual allocation unit address space in various embodiments. In each case, a range of addresses from the address space is assigned to each of the non-volatile solid-state storages in a storage cluster, or to each of the storage nodes in a storage cluster. The ranges may be non-overlapping, such that each non-volatile solid-state storage unit is assigned a range that differs from the ranges of the other non-volatile solid-state storage units. In this mechanism, no address from this address space repeats anywhere in the storage cluster. That is, each address from this address space is unique, and no two portions of user data are assigned the same address from this address space, during the expected lifespan of the system. Each time one of the addresses from this address space is assigned to a portion of user data in a non-volatile solid-state storage unit, whether the address is a medium address, a segment address, or a virtual allocation unit, the address (upon assignment according to the sequence) should be different from all such addresses previously assigned according to the sequence in that non-volatile solid-state storage unit. Thus, the addresses may be referred to as sequential and nonrepeating in this address space. The address space with these properties could include the medium address space, the segment address space and/or the virtual allocation unit address space. A non-volatile solid-state storage unit can allocate the assigned range of addresses in the non-volatile solid-state storage without synchronization with other non-volatile solid-state storage units in a storage cluster in some embodiments.
Each range of the address space has upper and lower bounds in some embodiments. Overall, the address space has a range that exceeds the likely maximum number of addresses from the address space that would be assigned during the expected lifespan of a system. In one embodiment, the sequential, nonrepeating addresses in the address space are binary numbers with at least 128 bits. The amount of bits may vary in embodiments, however with 128 bits, two raised to the 128th power is greater than the expected maximum number of addresses occurring for the lifetime of the system. The upper bound of the address space is greater than or equal to this number, or could include or be this number, in some embodiments. Larger numbers could be applied as technology further advances to higher operating speeds and lower time delays for reading and/or writing. The lower bound of the address space could be zero or one, or some other suitable low number, or negative numbers could be used.
Applying the sequential, nonrepeating addresses to one or more of the medium addresses, the segment addresses, or the virtual allocation units, enhance data recovery and flash writes. In some embodiments, the storage cluster, the storage node or the non-volatile, solid-state storage unit performs a snapshot of the present contents of the storage cluster, the storage node, or the non-volatile solid-state storage unit. At a later time, a particular version of user data can be recovered by referring to the snapshot. Since the relevant addresses do not have duplicates, there is an unambiguous record of the version of the user data at the time of the snapshot, and data is readily recovered if still existing in the relevant memory. Formats for snapshots are readily devised, and may include a file with a record of the contents of the cluster, the storage node, or the non-volatile solid-state storage unit, applying one or more address schemes. Depending on which address scheme or schemes is present in the snapshot, the address translation tables 502A, 502B, 502C, 502D, 502E can be applied to determine physical flash memory locations and presence or absence in the flash memory 206 of the desired data for recovery. It should be appreciated that various embodiments can apply various addressing schemes, with various numbers of address translations, various numbers of translation tables, various ranges for the addresses and various names for the addresses. Such address schemes may be developed for various reasons, such as performance, table size reduction, etc.
For flash writes, in some embodiments blocks of flash pages 224 are erased, and then individual flash pages 224 (see FIG. 3) are written in sequential order within a single erased block. This operation is supported by the above-described addressing mechanism, which assigns sequential addresses to data segments and/or data shards as they arrive for storage. In some embodiments, information relating to the medium address, the segment address, and/or the virtual allocation unit is written to a header of the flash page 224, thus identifying data stored in the flash page 224 (e.g., as data shards). The flash page 224, in such embodiments, becomes self-describing and self-checking, via the information in the header.
FIG. 6 is a multiple level block diagram, showing a controller 212, flash dies 222, and interior details of flash dies 222. Diagnostic information relating to the flash memory 206 can be obtained on a per flash package 602, per flash die 222, per flash plane 604, per flash block 606, and/or per flash page 224 basis across the entirety of a storage cluster 160, in some embodiments. In the example shown in FIG. 6, the flash memory 206 includes multiple flash packages 602. Each flash package 602 includes multiple flash dies 222, each of which in turn includes multiple flash planes 604. Each flash plane 604 includes multiple flash blocks 606 each of which in turn includes multiple flash pages 224. The diagnostic information is gathered or generated by the controller 212 of each non-volatile solid-state storage 152 and forwarded to the CPU 156 of the corresponding storage node 150. In some embodiments, the CPU 156 performs further analysis on the diagnostic information and generates further diagnostic information. The controller 212 and/or the CPU 156 can write the diagnostic information to a memory in the storage cluster 160, for example the flash memory 206 or the DRAM 216 of a non-volatile solid-state storage 152, the memory 154 coupled to the CPU 156 in a storage node 150, or other memory of the storage cluster 160, storage node 150, or non-volatile solid-state storage 152. The diagnostic information can be stored as metadata, in some embodiments. The DRAM 216 could be on-chip, e.g. on the controller 212, or off-chip, e.g., separate from and coupled to the controller 212, in various embodiments.
One type of diagnostic information is obtained by tracking bit errors per flash page 224 or per codeword. Each flash page 224 has multiple codewords, in some embodiments. Incidents of error correction could be reported and these incidents may be used as a source on which to base the diagnostic information. For example, the controller 212 could track bit errors of the flash memory 206 and forward the information about the bit errors to the CPU 156, which could then tabulate this and/or generate further diagnostic information. Bit errors, or error corrections, can be tracked from feedback from an error correction block 608 in the controller 212 in some embodiments. The CPU 156 or the controller 212 could track wear of flash blocks 606 in the flash memory 206, e.g., by establishing and updating a wear list in memory coupled as described above, responsive to or based on some of the diagnostic information. Such tracking could include ranking flash blocks 606 as to levels of wear, or comparing flash blocks 606 as to levels of wear. The flash memory 206 can be characterized over time, based on the diagnostic information. Characterization information could indicate changes or trends in the flash memory 206, such as increases in the rate of errors or error correction over time. This characterization can be performed at any of the levels of granularity discussed above.
In some embodiments, the CPU 156 sends the diagnostic information, or summarizes the diagnostic information in a report and sends the report, via a network. The diagnostic information or the report could be sent to an appropriate person or organization, which could include an owner or operator of a storage cluster 160, a manufacturer of storage nodes 150, a manufacturer of flash memory 206, flash packages 602 or flash dies 222 or other interested or authorized party. These reports could benefit the manufacturers, which can use the information for warranty service and/or to highlight manufacturing and reliability problems and guide improvements. The reports also benefit users, who can plan system maintenance, repairs and upgrades based on the details in the reports. Actual behavior of the flash memory 206 over time can be compared to predicted behavior or to warranties if applicable.
The CPU 156 or the controller 212 could make decisions based on the diagnostic information. For example, if it is determined that a flash block 606 has a high level of wear, the CPU 156 or the controller 212 could determine to write some of the user data to another flash block 606 with a lower level of wear. The controller 212 may bias a read from the flash memory, or a write to the flash memory 206, as a response to producing or obtaining the diagnostic information. Depending on the type of flash, and whether specific features are available on flash dies 222, this biasing can take different forms. Biasing the writes or the reads may extend the lifespan of some or all of the flash memory 206. For example, some types of flash dies 222 may support a variable write time, a variable write voltage, a variable read time, a variable reference voltage, a variable reference current or a variable number of reads. The controller 212 could determine, based on the diagnostic information, to direct a flash die 222 to apply a specified value of one of the above variable parameters to a specified write or read. The specified value could be applied to specified writes or reads to flash pages 224, flash blocks 606, flash dies 222, and/or flash packages 602. Thus, the granularity of the application of variable parameters to writes or reads of the flash memory 206 can match and be supported by the granularity of the diagnostic information itself.
Continuing with the above examples, the variable parameters are applicable to multiple scenarios. In a case where a flash block 606 is experiencing an increase in read errors, the controller 212 could direct the flash block 606 to perform repeated reads at differing reference voltages or reference currents. If a variable reference voltage or a reference current is not available, the controller 212 could perform the multiple reads without varying the reference voltage or current. The controller 212, or the CPU 156 could then perform statistical analysis of the reads and determine a most likely bit value for each read of data in the flash block 606. In cases where a variable write parameter is supported in flash dies 222, a value of a variable write parameter can be selected in an attempt to increase write or read reliability of the flash die 222. Similarly, in cases where a variable read parameter is supported in flash dies 222, a value of a variable read parameter can be selected in an attempt to increase read reliability of the flash die 222. In some embodiments a value for a variable write or read parameter could be selected in response to a determination that some portion of flash memory 206 has greater wear relative to another portion. As a further example, some types of flash dies 222 may have and support changing from multilevel cell (MLC) operation to single cell (SLC) operation. SLC flash has one bit per cell, and MLC flash has more than one bit per cell. The CPU 156 or the controller 212 could direct a flash die 222 to change from MLC operation to SLC operation in order to increase reliability of reads or writes. This change may be in response to determining that some portion of the flash memory 206 has greater wear relative to another portion.
FIG. 7 illustrates failure mapping, in which addresses are mapped around defects 714, 716 in flash memory 206, in some embodiments. Failure mapping can be applied to known defects and/or newly discovered defects. Application of failure mapping to known defects supports yield recovery by allowing a manufacturer to install flash packages 602 with known defective flash dies 222 into the flash memory 206 of a non-volatile solid-state storage 152 (see FIGS. 3 and 6). Dynamic application of failure mapping to newly discovered defects supports virtualizing the storage capacity of a storage cluster 160, use of all available storage space, and graceful degradation of storage capacity without catastrophic failure. Defects 714, 716 in flash memory 206 can be various sizes and encompass various ranges of addresses in physical address space 704. For example, a relatively small defect 714 could be a single failed bit at a single physical address. A relatively large defect 716 could be a failed flash page, flash block, flash die, or flash package, with a corresponding range of addresses in the physical address space 704. It should be appreciated that the physical address of a defect 714, 716 in the flash memory 206 is related to the physical address of the defect relative to the flash die, the physical address of the flash die relative to the flash package, and the physical address of the contents of the flash package relative to address decoding of flash packages in the flash memory 206 (e.g., address decoding on a printed circuit board and/or address decoding of multiple printed circuit boards relative to the non-volatile solid-state storage unit). Characterization of the defect 714, 716 as small or large is arbitrary and is for illustrative purposes, and further aspects of the defect 714, 716 may be characterized.
Still referring to FIG. 7, in order to perform failure mapping, the non-volatile solid-state storage unit determines which addresses in a physical address space 704 are usable addresses 710, and which addresses in the physical address space 704 are unusable addresses 712. The unusable addresses 712 correspond to locations of the defects 714, 716 in the flash memory 206, and the usable addresses 710 correspond to locations in the flash memory 206 that have working, non-defective flash bits. The usable addresses 710 and unusable addresses 712 are mutually exclusive in some embodiments. That is, the usable addresses 710 in the physical address space 704 exclude the unusable addresses 712 and thus exclude the physical addresses of the defects 714, 716 in the flash memory 206. In some embodiments, the non-volatile solid-state storage 152 generates a defects map 708 that indicates the unusable addresses 712 in the physical address space 704 or otherwise indicates the defects 714, 716 in the flash memory 206. Various formats for the defects map 708, such as a list, an array, a table or a database, are readily devised in accordance with the teachings herein.
A mapper 706 in the non-volatile solid-state storage unit maps memory addresses of a memory address space 702 into physical addresses in the physical address space 704. More specifically, the mapper 706 maps the memory addresses of the memory address space 702 into the usable addresses 710 of the physical address space 704, and thereby maps around or bypasses the unusable addresses 712 and corresponding defects 714, 716 of the flash memory 206. The mapper 706 is thus based on the defects 714, 716, and in some embodiments is based on the defects map 708. The mapper 706 could include one or more of the address translation tables 502 (see FIG. 5), which translate addresses of the user data, at various levels and in various address spaces in the system, to physical memory locations. For example, one embodiment of the non-volatile solid-state storage unit generates an address translation table 502 (e.g., address translation table 502E) that maps around defects 714, 716 in the flash memory 206 on a per flash package 602, flash die 222, flash plane 604, flash block 606, flash page 224 or physical address basis (see FIG. 6). The address translation table 502 is applied to write and read accesses of user data.
There are multiple mechanisms in which a storage node or non-volatile solid-state storage unit could determine the defects 714, 716 in flash memory 206. In embodiments making use of flash dies or flash packages with defects 714, 716 known at the time of assembly of a non-volatile solid-state storage unit, storage node or storage cluster, information could be provided from an external source. The storage nodes could determine the usable addresses 710 and unusable addresses 712 of the flash memory 206 based on information from a manufacturer or tester of flash packages, or flash dies, or a tester of the flash memory 206. This information could be provided via a network in some embodiments. The storage nodes support yield recovery of flash packages that have been downgraded as a result of testing. Downgraded flash packages may have known defects such as a mixture of non-defective flash dies and defective flash dies, or may have flash dies with one or more defective flash blocks or other defective portions of flash memory 206. In embodiments utilizing the dynamic determination of defects 714, 716, the controller of a non-volatile solid-state storage and/or the CPU 156 of a storage node could determine defects 714, 716 as part of or based on the diagnostic information described above with reference to FIG. 6. For example, a threshold could be established for error counts, error rates, error correction counts or error correction rates. When a portion of the flash memory 206 exceeds a total error count, error rate, error correction count or error correction rate, that portion of the flash memory 206 could be declared defective and mapped out as described above. Defect detection, defect mapping, and address translation to map around the defects can be performed on an ongoing basis.
FIG. 8A illustrates a flash block 606 having defects in a flash die of a flash memory. Flash block 606 is an example of a type of defect that can be mapped around by the mapper 706 as discussed above regarding FIG. 7. Further mechanisms discussed below with reference to FIGS. 8A-8C are applicable to this and other types of defects, in some embodiments of non-volatile solid-state storage units. In FIG. 8A, the flash block 606 has several stuck or faulty bit lines 802. One or more stuck bit lines 802 is a type of defect seen in some flash dies. A flash page 224 is seen as defective, since one or more bit locations (shown in the diagram as having an “X”) have bit values corresponding to the stuck bit lines 802. A bit line could be stuck at a “0” or a “1” value for these bit locations. Flash page 224 is not limited to a stuck bit line defect as other defects besides a stuck bit line may be integrated with the embodiments. Flash block 606 has a spare data area 804, which can be applied for various purposes. Bits from the spare data area 804 may be applied as parity bits for pages 224. Bits from the spare data area 804 may also be used for repairing data read from a defective flash block 606, in some embodiments. One known type of flash die employs a flash page size of about 16,384 bytes and a spare data area size of about 1,216 bytes. Some embodiments of the non-volatile solid-state storage unit use the spare data area 804 to store replacement data. Some embodiments use the spare data area 804 to store ordered lists (tuples) of pointers into a page and a replacement page. These pointers are associated with or are part of a mask 806, in various embodiments.
FIG. 8B shows a mask 806 with a defect indicator 808, which can be applied to mask a defect in the flash block 606 of FIG. 8A. Various embodiments of the non-volatile solid-state storage unit can determine that a flash block 606 has a defect, and generate a mask 806 that indicates the defect of flash block 606. This determining and mask generation could be based on manufacturer information, such as provided from a manufacturer of flash dies or flash packages. In some embodiments, the determining and mask generation could be based on characterizing the flash memory during or after assembly of a non-volatile solid-state storage 152. Characterizing may include multiple writes and reads from the flash memory. The determining and mask generation could also be based on ongoing monitoring during system operation, e.g., the diagnostic information discussed with reference to FIG. 6. The non-volatile solid-state storage unit can generate a mask 806 at various times during the lifespan of a storage cluster. In some embodiments, the non-volatile solid-state storage unit stores the mask 806 as metadata.
The example mask 806 shown in FIG. 8B has a defect indicator 808, in the form of a value at each bit location corresponding to a defect, e.g., a defect in a flash page 224. The defect indicator 808 aligns with the defect in the flash page 224. Various formats for masks 806 are readily devised in accordance with the teachings herein. For example, the mask 806 could indicate non-defective bits in a flash page read by having a specified value in bit locations in the mask 806 corresponding to the non-defective bits in the flash page 224. The mask 806 may indicate defective bits in a flash page read by having a further specified value in bit locations in the mask 806 corresponding to the defective bits in the flash page 224. These specified values could be weights, codes, flags, or other types of guides for mask operations. Various schemes for indicating defective or non-defective bits, and associated values, are readily devised in accordance with the teachings herein. In some embodiments mask 806 is a bit mask or mask 806 is a set of one or more pointers to bit locations. Defect indicator 808 may be interpreted by the non-volatile solid-state storage unit as pointing to a bit location in a flash page 224 that is a don't care. In some embodiments the defect indicator 808 may be interpreted as pointing to a bit location in a flash page 224 where a bit value should be deleted or removed, where a bit value should be replaced or where a bit value is likely stuck. Since the controller 212 (see FIGS. 3, 4 and 6) has access to the physical flash addresses, masks 806 can be created that apply to pages 224, blocks 606, planes 604, dies 222, etc. The ability to create masks in this manner amortizes the mask overhead accordingly. Some embodiments can create a mask hierarchy to reduce overhead. In a mask hierarchy, masks can be combined for different levels (block, plane, die, etc.) to generate one or more page masks, e.g., page specific masks. These could be pre-calculated, or generated on-the-fly (i.e., as needed) in some embodiments.
FIG. 8C shows various strategies and mechanisms for applying the mask 806 shown in FIG. 8B to reads of data in the flash block 606 of FIG. 8A. Various embodiments of the non-volatile solid-state storage units may apply one or more of these strategies and mechanisms, however, the embodiments are not limited to the example strategies. Variations of these strategies and mechanisms could be applied to portions of data other than a flash page 224, although application to a flash page 224 is illustrated since many types of flash memory support page reads. A first strategy is to replace bit values in the data resulting from reading the flash page 224A from the flash block 606. In keeping with the example flash block 606 having defects, the flash page 224A has multiple stuck bits corresponding to the stuck bit lines 802. In the first strategy, bit values of defective bits, i.e., bit locations in the flash page 224A corresponding to the stuck bit lines 802, are replaced with replacement bit values from the spare data area 804 of the flash block 606. The mask 806 indicates which bit locations in the flash page 224A are defective and receives the replacement bit values to repair the data resulting from the read of the flash page 224A. If two blocks have similar bit line failures, some embodiments can combine these and generate a superset of bit line failures, reducing the number of masks 806 and sacrificing some pre-failed bits, as long as space is available to do so.
A second strategy is to replace the flash page 224 with replacement data from the spare data area 804 which has been set aside to act as a replacement flash page. In this strategy, the mask 806 has a defect indicator 808 that indicates the entire flash page 224B of a particular flash block 606 should be replaced. A third strategy is to remove defective bit values from a flash page 224C and provide substitute bit values from the spare data area 804. Similar to the first strategy, the mask 806 indicates which bit locations in the flash page 224C are defective and should be removed. In contrast to the first strategy, in which replacement bit values are inserted into the same bit locations in the flash page 224A as the defective bit values being replaced, the substitute data portion 810E is inserted at the least significant bit (LSB) end of the flash page 224C. Data portions 810B, 810C, 810D are shifted, e.g., to the left or towards the most significant bit (MSB) end of the flash page 224C. This action stitches the gaps produced by removing the defective bit values from the flash page 224C, i.e., removing the defective bit values from the data read from the flash page 224C. The resultant repaired flash page 224D has the various data portions 810A, 810B, 810C, 810D, 810E as contiguous data bits with no gaps, to repair the data resulting from the read of the flash page 224C. A variation of the third strategy is to insert the substitute data portion 810E at the MSB end of the flash page 224C. A further variation is to insert the substitute data portion 810E elsewhere in the flash page 224C. Data portions 810A, 810B, 810C, 810D are shifted accordingly, in these variations. The substitute data portion 810E is from the spare data area 804 of the flash block 606. The repaired flash page 224D can be substituted for a defective page from the flash block 606, as a replacement flash page.
With reference back to FIG. 6 and ongoing reference to FIGS. 8A-8C, one embodiment of the controller 212 of a non-volatile solid-state storage unit has an error correction block 608 that applies Log-Likelihood-Ratio (LLR) techniques to data read from the flash block 606, with application of the mask 806. Log-Likelihood-Ratio is defined as LLR=log 10(P(X=0|Y)/P(X=1|Y)), where P(X=0|Y) is the conditional probability that a bit was written as “0” if the value read is Y, and similarly for P(x=1|Y). Y can be binary {0,1} in the case of a hard read, or can have more values in case of a soft read. The set of values depends on the number of performed reads (i.e., resolution) from the flash memory 206. The LLRs are calculated based on the hard or soft value read from flash using a transfer curve/look-up table. The transfer curve/look-up table can be obtained through a characterization process during or before production, or the transfer curve/look-up table can be obtained dynamically during operation. Tables/transfer curves are calculated by estimation of the conditional probabilities in the LLR definition. To estimate the conditional probabilities, known data is written to the part and hard/soft data is read from part. Conditional probability is estimated from the collected data. In the case of the dynamic estimation, data is read, either hard or soft, and decoded with the “a priori LLR”. In the simplest case of the hard read, an equal LLR of opposite sign is assigned to 0's and 1's. Then, the decoded sequence is used in lieu of the known sequence for the conditional probability estimation.
One of the failure mechanisms of NAND flash is that bit lines 802 can be stuck, as described above with reference to FIG. 8A. With a stuck bit line 802, a transistor is stuck in an open or closed position and affects all the bits on the same bit line 802 in the flash block 606. The stuck bits can be detected in several techniques. One mechanism is to read a number of pages 224 from the flash block 606, e.g., N pages 224. The data obtained from read is mapped as 1→−1, 0→1 and accumulated. The accumulated value is decoded such that if accumulated value z for a position is −N/2<=z<=N/2 position is set to 0, if z<−N/2 or z>N/2 position is set to 1. After detection, position set to 1 denotes the position of stuck bits. The stuck bit can be “0” or “1” with equal probability, so the LLR assigned to these positions is LLR=0. This information is beneficial for decoding performance both using BCH (Bose, Chaudhuri, and Hocquenghem) errors and erasures decoding or LDPC decoding, as having no information is preferred to having wrong information. This allows the decode operation to determine what the true value should be without bias. A value of LLR=0 is conveying that “0” or “1” are equally likely. The decode operation can then be performed as normal. Masking, through the use of the mask 806, provides the additional information to replace the bad bits. The decode operation of the stuck position doesn't matter because the stuck position bit would be replaced with another bit as described above, in various embodiments.
FIG. 9 is a flow diagram of a method for masking defective bits in a storage array, which can be practiced on or by the storage cluster, storage nodes and/or non-volatile solid-state storage units in accordance with some embodiments. Some or all of the actions of the method can be practiced by a processor, such as a controller of a non-volatile solid-state storage or a CPU of a storage node. User data and metadata are distributed throughout storage nodes of a storage cluster, in an action 902. The user data is accessible via erasure coding from the storage nodes even if two of the storage nodes are unreachable. Each of the storage nodes has non-volatile solid-state storage with flash memory. The storage nodes may be housed within a single chassis that couples the storage nodes of the storage cluster. A defect is identified in a flash block, in an action 904. The defect could be determined or identified based on externally provided information such as from a manufacturer or test results. The defect could be determined based on processes internal to the storage cluster, storage nodes and non-volatile solid-state storage units, such as tracking errors or error corrections and generating diagnostic information. A mask is generated, in an action 906. The mask indicates the flash block and the defect by pointing to or otherwise indicating bits or bit locations associated with the flash block. A flash page is read, in an action 908. The mask is applied, in an action 910. Application of the mask could be performed in various ways, and results in data that has defective bits masked accordingly. Some or all of the above actions are repeated as further defects are determined and further masks or updates to the mask are performed. In further embodiments of the method, the mask could indicate other portions of the flash or non-volatile memory, such as the page, die or package. In still further embodiments, the mask could be an hierarchical mask set as described above.
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. 10 is an illustration showing an exemplary computing device which may implement the embodiments described herein. The computing device of FIG. 10 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) 1001, which is coupled through a bus 1005 to a memory 1003, and mass storage device 1007. Mass storage device 1007 represents a persistent data storage device such as a disc drive, which may be local or remote in some embodiments. The mass storage device 1007 could implement a backup storage, in some embodiments. Memory 1003 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 1003 or mass storage device 1007 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 1001 may be embodied in a general-purpose processor, a special purpose processor, or a specially programmed logic device in some embodiments.
Display 1011 is in communication with CPU 1001, memory 1003, and mass storage device 1007, through bus 1005. Display 1011 is configured to display any visualization tools or reports associated with the system described herein. Input/output device 1009 is coupled to bus 1005 in order to communicate information in command selections to CPU 1001. It should be appreciated that data to and from external devices may be communicated through the input/output device 1009. CPU 1001 can be defined to execute the functionality described herein to enable the functionality described with reference to FIGS. 1-9. The code embodying this functionality may be stored within memory 1003 or mass storage device 1007 for execution by a processor such as CPU 1001 in some embodiments. The operating system on the computing device may be MS-WINDOWSTM, UNIXTM, LINUXTM, iOSTM, CentOSTM, AndroidTM, Redhat LinuxTM, z/OSTM, 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.

## Claims

1. A method of failure mapping, comprising:
distributing user data throughout a plurality of storage nodes through erasure coding, wherein each of the plurality of storage nodes has a non-volatile solid-state storage with non-volatile memory;
determining that the non-volatile memory has a defect;
generating a mask that indicates the defect, the generating based on error correction of reads of the non-volatile solid-state storage, wherein the mask is one of a plurality of masks in a mask hierarchy in the storage cluster, wherein the mask hierarchy includes masks of differing physical levels of the non-volatile memory, and wherein the generating supports combining the masks of differing physical levels to generate one or more page specific masks; and
reading from the non-volatile memory with application of the mask, wherein the reading and the application of the mask are performed by the non-volatile solid-state storage.

2. The method of claim 1, wherein application of the mask comprises:
substituting of a replacement page for a defective page from the non-volatile memory.

3. The method of claim 1, wherein application of the mask comprises:
weighting a bit value corresponding to a stuck bit line, such that an error correction interprets the weighted bit value as a don't care.

4. The method of claim 1, wherein application of the mask comprises:
assigning a log-likelihood-ratio (LLR) value of zero to a bit location corresponding to a stuck bit line, such that a decoding interprets the bit location as equally likely to be stuck at zero or one.

5. The method of claim 1, wherein application of the mask comprises:
combining a plurality of masks to form the mask, wherein the mask is specific to a page; and
masking at least one bit of the page, in accordance with the mask.

6. The method of claim 1, wherein application of the mask comprises:
replacing a first bit value corresponding to a faulty bit line, by a second bit value from a spare data area in the non-volatile memory.

7. The method of claim 1, wherein reading from the non-volatile memory, with application of the mask comprises:
reading one of a plurality of pages affected by a faulty bit line;
removing, from data produced by the reading, a first bit value corresponding to the faulty bit line;
inserting a second bit value, from a spare data area in the non-volatile memory, into a least significant bit (LSB) location or a most significant bit (MSB) location of the data produced by the reading; and
shifting a portion of the data produced by the reading so as to produce aligned data with the first bit value so removed and the second bit value so inserted.

8. The method of claim 1, wherein the determining comprises one of:
interpreting manufacturing information regarding the non-volatile memory;
characterizing the non-volatile memory, with a portion of the non-volatile memory assembled in each of the plurality of storage nodes; or
analyzing diagnostic information produced during operation of the plurality of storage nodes.

9. A plurality of storage nodes in a chassis, comprising:
each of the plurality of storage nodes having a processor and a non-volatile solid-state storage having non-volatile memory for user data storage, each non-volatile solid-state storage having a controller;
the plurality of storage nodes configured to distribute the user data and metadata associated with the user data throughout the plurality of storage nodes;
the non-volatile solid-state storage configured to generate a plurality of masks in a mask hierarchy that includes masks of differing physical levels of the non-volatile memory, and supports combining the masks of differing physical levels to generate one or more page specific masks, such that each of the plurality of masks indicates the non-volatile memory has a defect;
the non-volatile solid-state storage configured to apply one of the plurality of masks in a read from the non-volatile memory; and
the non-volatile solid-state storage configured to produce data with one of a replacement bit value, a weighted value, or a don't care bit value, substituting for a defective bit value as a result of applying the one of the plurality of masks in the read from the non-volatile memory.

10. The plurality of storage nodes in a chassis of claim 9, further comprising:
the plurality of storage nodes configured to determine that the non-volatile memory has a defective page or a faulty bit line across a block, die, or package, based on manufacturer information.

11. The plurality of storage nodes in a chassis of claim 9, further comprising:
the plurality of storage nodes configured to determine that the non-volatile memory of the non-volatile solid-state storage of one of the plurality of storage nodes has a defective page or a faulty bit line, based on error correction of reads of the non-volatile solid-state storage.

12. The plurality of storage nodes in a chassis of claim 9, further comprising:
the plurality of storage nodes configured to determine that the non-volatile memory of the non-volatile solid-state storage of one of the plurality of storage nodes has a defective page or a faulty bit line, based on diagnostic information produced by the plurality of storage nodes.

13. The plurality of storage nodes in a chassis of claim 9, wherein the replacement bit value is provided from a spare data area in the non-volatile memory.

14. The plurality of storage nodes in a chassis of claim 9, wherein the defective bit value is removed from a first location in the data and the replacement bit value is substituted in a second location in the data, with a portion of the data shifted to accommodate the replacement bit value.

15. A storage cluster, comprising:
a plurality of storage nodes, each of the plurality of storage nodes having a processor and non-volatile solid-state storage with non-volatile memory for storage of user data;
the plurality of storage nodes configured to distribute the user data and metadata associated with the user data throughout the plurality of storage nodes;
the non-volatile solid-state storage configured to determine that a portion of the non-volatile memory has a defect based on error correction of reads of the non-volatile solid-state storage;
the non-volatile solid-state storage configured to generate a mask that indicates the defect, and read from the non-volatile memory the portion having the defect with application of the mask; and
the non-volatile solid-state storage configured to generate further masks that indicate further defects of the non-volatile memory, wherein the mask and the further masks are in a mask hierarchy that includes masks of differing physical levels of the non-volatile memory, and supports combining the masks of differing physical levels to generate one or more page specific masks.

16. The storage cluster of claim 15, further comprising:
the non-volatile solid-state storage having an error correction block configured to interpret weights on bit locations in data resulting from the read from the non-volatile memory, wherein the mask is configured to indicate a faulty bit line by weighting a bit in the data resulting from the read from the non-volatile memory.

17. The storage cluster of claim 15, wherein:
the non-volatile memory includes a die that is known as defective at a time of assembly of the non-volatile memory;
the non-volatile solid-state storage is configured to receive manufacturer information regarding the die that is known defective; and
the non-volatile solid-state storage is configured to generate the mask based at least in part on the manufacturer information.

18. The storage cluster of claim 15, further comprising:
the non-volatile solid-state storage configured to store the mask as metadata in a memory coupled to the non-volatile solid-state storage.

19. The storage cluster of claim 15, further comprising:
the non-volatile solid-state storage configured to substitute a replacement page for a defective page.

20. The storage cluster of claim 15, further comprising:
the non-volatile solid-state storage configured to delete, in data from the read from the non-volatile memory, a first bit value resulting from a faulty bit line; and
the non-volatile solid-state storage configured to insert, into the data with the deleted first bit value, a second bit value from a spare data area in the non-volatile memory.