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ZPOOLCONCEPTS(7) Miscellaneous Information Manual ZPOOLCONCEPTS(7) NAME zpoolconcepts -- overview of ZFS storage pools DESCRIPTION Virtual Devices (vdevs) A "virtual device" describes a single device or a collection of devices, organized according to certain performance and fault characteristics. The following virtual devices are supported: disk A block device, typically located under /dev. ZFS can use indi- vidual slices or partitions, though the recommended mode of opera- tion is to use whole disks. A disk can be specified by a full path, or it can be a shorthand name (the relative portion of the path under /dev). A whole disk can be specified by omitting the slice or partition designation. For example, sda is equivalent to /dev/sda. When given a whole disk, ZFS automatically labels the disk, if necessary. file A regular file. The use of files as a backing store is strongly discouraged. It is designed primarily for experimental purposes, as the fault tolerance of a file is only as good as the file sys- tem on which it resides. A file must be specified by a full path. mirror A mirror of two or more devices. Data is replicated in an identi- cal fashion across all components of a mirror. A mirror with N disks of size X can hold X bytes and can withstand N-1 devices failing, without losing data. raidz, raidz1, raidz2, raidz3 A distributed-parity layout, similar to RAID-5/6, with improved distribution of parity, and which does not suffer from the RAID-5/6 "write hole", (in which data and parity become inconsistent after a power loss). Data and parity is striped across all disks within a raidz group, though not necessarily in a consistent stripe width. A raidz group can have single, double, or triple parity, meaning that the raidz group can sustain one, two, or three failures, re- spectively, without losing any data. The raidz1 vdev type speci- fies a single-parity raidz group; the raidz2 vdev type specifies a double-parity raidz group; and the raidz3 vdev type specifies a triple-parity raidz group. The raidz vdev type is an alias for raidz1. A raidz group with N disks of size X with P parity disks can hold approximately (N-P)*X bytes and can withstand P devices failing without losing data. The minimum number of devices in a raidz group is one more than the number of parity disks. The recom- mended number is between 3 and 9 to help increase performance. draid, draid1, draid2, draid3 A variant of raidz that provides integrated distributed hot spares, allowing for faster resilvering, while retaining the bene- fits of raidz. A dRAID vdev is constructed from multiple internal raidz groups, each with D data devices and P parity devices. These groups are distributed over all of the children in order to fully utilize the available disk performance. Unlike raidz, dRAID uses a fixed stripe width (padding as neces- sary with zeros) to allow fully sequential resilvering. This fixed stripe width significantly affects both usable capacity and IOPS. For example, with the default D=8 and 4 KiB disk sectors the minimum allocation size is 32 KiB. If using compression, this relatively large allocation size can reduce the effective compres- sion ratio. When using ZFS volumes (zvols) and dRAID, the default of the volblocksize property is increased to account for the allo- cation size. If a dRAID pool will hold a significant amount of small blocks, it is recommended to also add a mirrored special vdev to store those blocks. In regards to I/O, performance is similar to raidz since, for any read, all D data disks must be accessed. Delivered random IOPS can be reasonably approximated as floor((N-S)/(D+P))*single_drive_IOPS. Like raidz, a dRAID can have single-, double-, or triple-parity. The draid1, draid2, and draid3 types can be used to specify the parity level. The draid vdev type is an alias for draid1. A dRAID with N disks of size X, D data disks per redundancy group, P parity level, and S distributed hot spares can hold approximately (N-S)*(D/(D+P))*X bytes and can withstand P devices failing without losing data. draid[parity][:datad][:childrenc][:sparess] A non-default dRAID configuration can be specified by appending one or more of the following optional arguments to the draid key- word: parity The parity level (1-3). data The number of data devices per redundancy group. In general, a smaller value of D will increase IOPS, improve the compression ratio, and speed up resilvering at the expense of total usable capacity. Defaults to 8, unless N-P-S is less than 8. children The expected number of children. Useful as a cross- check when listing a large number of devices. An error is returned when the provided number of children dif- fers. spares The number of distributed hot spares. Defaults to zero. spare A pseudo-vdev which keeps track of available hot spares for a pool. For more information, see the "Hot Spares" section. log A separate intent log device. If more than one log device is specified, then writes are load-balanced between devices. Log de- vices can be mirrored. However, raidz vdev types are not sup- ported for the intent log. For more information, see the "Intent Log" section. dedup A device solely dedicated for deduplication tables. The redun- dancy of this device should match the redundancy of the other nor- mal devices in the pool. If more than one dedup device is speci- fied, then allocations are load-balanced between those devices. special A device dedicated solely for allocating various kinds of internal metadata, and optionally small file blocks. The redundancy of this device should match the redundancy of the other normal de- vices in the pool. If more than one special device is specified, then allocations are load-balanced between those devices. For more information on special allocations, see the "Special Allocation Class" section. cache A device used to cache storage pool data. A cache device cannot be configured as a mirror or raidz group. For more information, see the "Cache Devices" section. Virtual devices cannot be nested arbitrarily. A mirror, raidz or draid virtual device can only be created with files or disks. Mirrors of mirrors or other such combinations are not allowed. A pool can have any number of virtual devices at the top of the configura- tion (known as "root vdevs"). Data is dynamically distributed across all top-level devices to balance data among devices. As new virtual devices are added, ZFS automatically places data on the newly available devices. Virtual devices are specified one at a time on the command line, separated by whitespace. Keywords like mirror and raidz are used to distinguish where a group ends and another begins. For example, the following creates a pool with two root vdevs, each a mirror of two disks: # zpool create mypool mirror sda sdb mirror sdc sdd Device Failure and Recovery ZFS supports a rich set of mechanisms for handling device failure and data corruption. All metadata and data is checksummed, and ZFS automatically repairs bad data from a good copy, when corruption is detected. In order to take advantage of these features, a pool must make use of some form of redundancy, using either mirrored or raidz groups. While ZFS sup- ports running in a non-redundant configuration, where each root vdev is simply a disk or file, this is strongly discouraged. A single case of bit corruption can render some or all of your data unavailable. A pool's health status is described by one of three states: online, degraded, or faulted. An online pool has all devices operating normally. A degraded pool is one in which one or more devices have failed, but the data is still available due to a redundant configuration. A faulted pool has corrupted metadata, or one or more faulted devices, and insufficient replicas to continue functioning. The health of the top-level vdev, such as a mirror or raidz device, is po- tentially impacted by the state of its associated vdevs or component de- vices. A top-level vdev or component device is in one of the following states: DEGRADED One or more top-level vdevs is in the degraded state because one or more component devices are offline. Sufficient replicas exist to continue functioning. One or more component devices is in the degraded or faulted state, but sufficient replicas exist to continue functioning. The underlying conditions are as follows: * The number of checksum errors or slow I/Os exceeds acceptable levels and the device is degraded as an indication that some- thing may be wrong. ZFS continues to use the device as nec- essary. * The number of I/O errors exceeds acceptable levels. The de- vice could not be marked as faulted because there are insuf- ficient replicas to continue functioning. FAULTED One or more top-level vdevs is in the faulted state because one or more component devices are offline. Insufficient replicas ex- ist to continue functioning. One or more component devices is in the faulted state, and insuf- ficient replicas exist to continue functioning. The underlying conditions are as follows: * The device could be opened, but the contents did not match expected values. * The number of I/O errors exceeds acceptable levels and the device is faulted to prevent further use of the device. OFFLINE The device was explicitly taken offline by the zpool offline com- mand. ONLINE The device is online and functioning. REMOVED The device was physically removed while the system was running. Device removal detection is hardware-dependent and may not be supported on all platforms. UNAVAIL The device could not be opened. If a pool is imported when a de- vice was unavailable, then the device will be identified by a unique identifier instead of its path since the path was never correct in the first place. Checksum errors represent events where a disk returned data that was ex- pected to be correct, but was not. In other words, these are instances of silent data corruption. The checksum errors are reported in zpool status and zpool events. When a block is stored redundantly, a damaged block may be reconstructed (e.g. from raidz parity or a mirrored copy). In this case, ZFS reports the checksum error against the disks that contained dam- aged data. If a block is unable to be reconstructed (e.g. due to 3 disks being damaged in a raidz2 group), it is not possible to determine which disks were silently corrupted. In this case, checksum errors are reported for all disks on which the block is stored. If a device is removed and later re-attached to the system, ZFS attempts to bring the device online automatically. Device attachment detection is hardware-dependent and might not be supported on all platforms. Hot Spares ZFS allows devices to be associated with pools as "hot spares". These de- vices are not actively used in the pool. But, when an active device fails, it is automatically replaced by a hot spare. To create a pool with hot spares, specify a spare vdev with any number of devices. For example, # zpool create pool mirror sda sdb spare sdc sdd Spares can be shared across multiple pools, and can be added with the zpool add command and removed with the zpool remove command. Once a spare re- placement is initiated, a new spare vdev is created within the configura- tion that will remain there until the original device is replaced. At this point, the hot spare becomes available again, if another device fails. If a pool has a shared spare that is currently being used, the pool cannot be exported, since other pools may use this shared spare, which may lead to potential data corruption. Shared spares add some risk. If the pools are imported on different hosts, and both pools suffer a device failure at the same time, both could attempt to use the spare at the same time. This may not be detected, resulting in data corruption. An in-progress spare replacement can be canceled by detaching the hot spare. If the original faulted device is detached, then the hot spare as- sumes its place in the configuration, and is removed from the spare list of all active pools. The draid vdev type provides distributed hot spares. These hot spares are named after the dRAID vdev they're a part of (draid1-2-3 specifies spare 3 of vdev 2, which is a single parity dRAID) and may only be used by that dRAID vdev. Otherwise, they behave the same as normal hot spares. Spares cannot replace log devices. Intent Log The ZFS Intent Log (ZIL) satisfies POSIX requirements for synchronous transactions. For instance, databases often require their transactions to be on stable storage devices when returning from a system call. NFS and other applications can also use fsync(2) to ensure data stability. By de- fault, the intent log is allocated from blocks within the main pool. How- ever, it might be possible to get better performance using separate intent log devices such as NVRAM or a dedicated disk. For example: # zpool create pool sda sdb log sdc Multiple log devices can also be specified, and they can be mirrored. See the "EXAMPLES" section for an example of mirroring multiple log devices. Cache Devices Devices can be added to a storage pool as "cache devices". These devices provide an additional layer of caching between main memory and disk. For read-heavy workloads, where the working set size is much larger than what can be cached in main memory, using cache devices allows much more of this working set to be served from low latency media. Using cache devices pro- vides the greatest performance improvement for random read-workloads of mostly static content. To create a pool with cache devices, specify a cache vdev with any number of devices. For example: # zpool create pool sda sdb cache sdc sdd Cache devices cannot be mirrored or part of a raidz configuration. If a read error is encountered on a cache device, that read I/O is reissued to the original storage pool device, which might be part of a mirrored or raidz configuration. The content of the cache devices is persistent across reboots and restored asynchronously when importing the pool in L2ARC (persistent L2ARC). This can be disabled by setting l2arc_rebuild_enabled=0. For cache devices smaller than 1 GiB, ZFS does not write the metadata structures required for rebuilding the L2ARC, to conserve space. This can be changed with l2arc_rebuild_blocks_min_l2size. The cache device header (512 B) is up- dated even if no metadata structures are written. Setting l2arc_headroom=0 will result in scanning the full-length ARC lists for cacheable content to be written in L2ARC (persistent ARC). If a cache device is added with zpool add, its label and header will be overwritten and its contents will not be restored in L2ARC, even if the device was previously part of the pool. If a cache device is onlined with zpool online, its contents will be restored in L2ARC. This is useful in case of memory pressure, where the contents of the cache device are not fully restored in L2ARC. The user can off- and online the cache device when there is less memory pressure, to fully restore its contents to L2ARC. Pool checkpoint Before starting critical procedures that include destructive actions (like zfs destroy), an administrator can checkpoint the pool's state and, in the case of a mistake or failure, rewind the entire pool back to the check- point. Otherwise, the checkpoint can be discarded when the procedure has completed successfully. A pool checkpoint can be thought of as a pool-wide snapshot and should be used with care as it contains every part of the pool's state, from proper- ties to vdev configuration. Thus, certain operations are not allowed while a pool has a checkpoint. Specifically, vdev removal/attach/detach, mirror splitting, and changing the pool's GUID. Adding a new vdev is supported, but in the case of a rewind it will have to be added again. Finally, users of this feature should keep in mind that scrubs in a pool that has a check- point do not repair checkpointed data. To create a checkpoint for a pool: # zpool checkpoint pool To later rewind to its checkpointed state, you need to first export it and then rewind it during import: # zpool export pool # zpool import --rewind-to-checkpoint pool Note that rewinding to a checkpoint will permanently discard it. Once the pool has been successfully imported with the above rewind command, you can- not rewind to the same checkpoint. To discard the checkpoint from a pool: # zpool checkpoint -d pool Dataset reservations (controlled by the reservation and refreservation properties) may be unenforceable while a checkpoint exists, because the checkpoint is allowed to consume the dataset's reservation. Finally, data that is part of the checkpoint but has been freed in the current state of the pool won't be scanned during a scrub. Special Allocation Class Allocations in the special class are dedicated to specific block types. By default, this includes all metadata, the indirect blocks of user data, in- tent log (in absence of separate log device), and deduplication tables. The class can also be provisioned to accept small file blocks or zvol blocks on a per dataset granularity. A pool must always have at least one normal (non-dedup/-special) vdev be- fore other devices can be assigned to the special class. If the special class becomes full, then allocations intended for it will spill back into the normal class. Deduplication tables can be excluded from the special class by unsetting the zfs_ddt_data_is_special ZFS module parameter. Inclusion of small file or zvol blocks in the special class is opt-in. Each dataset can control the size of small file blocks allowed in the spe- cial class by setting the special_small_blocks property to nonzero. See zfsprops(7) for more info on this property. FreeBSD ports 15.quarterly August 6, 2025 ZPOOLCONCEPTS(7)
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