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 individual slices or
             partitions, though the recommended mode of operation 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 system of which it is a part.  A file must be specified by
             a full path.

     mirror  A mirror of two or more devices.  Data is replicated in an identical 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 before data integrity is compromised.

     raidz, raidz1, raidz2, raidz3
             A variation on RAID-5 that allows for better distribution of parity and eliminates
             the RAID-5 "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.

             A raidz group can have single-, double-, or triple-parity, meaning that the raidz
             group can sustain one, two, or three failures, respectively, without losing any
             data.  The raidz1 vdev type specifies 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 device(s) failing before data integrity is
             compromised.  The minimum number of devices in a raidz group is one more than the
             number of parity disks.  The recommended number is between 3 and 9 to help increase
             performance.

     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 devices can be mirrored.  However, raidz
             vdev types are not supported for the intent log.  For more information, see the
             Intent Log section.

     dedup   A device dedicated solely for deduplication tables.  The redundancy of this device
             should match the redundancy of the other normal devices in the pool. If more than
             one dedup device is specified, 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 devices 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, so a mirror or raidz virtual device can only contain files
     or disks.  Mirrors of mirrors (or other combinations) are not allowed.

     A pool can have any number of virtual devices at the top of the configuration (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.
     The keywords mirror and raidz are used to distinguish where a group ends and another begins.
     For example, the following creates 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 supports 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 mirror or raidz device, is potentially impacted by
     the state of its associated vdevs, or component devices.  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 exceeds acceptable levels and the device is
                   degraded as an indication that something may be wrong.  ZFS continues to use
                   the device as necessary.

               ·   The number of I/O errors exceeds acceptable levels.  The device could not be
                   marked as faulted because there are insufficient 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 exist to continue functioning.

               One or more component devices is in the faulted state, and insufficient 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 command.

     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 device 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.

     If a device is removed and later re-attached to the system, ZFS attempts to put the device
     online automatically.  Device attach 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 devices 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 replacement is initiated, a new spare
     vdev is created within the configuration 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 can not 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 cancelled by detaching the hot spare.  If the
     original faulted device is detached, then the hot spare assumes its place in the
     configuration, and is removed from the spare list of all active pools.

     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 default, the intent log is allocated from blocks within the main pool.
     However, 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.

     Log devices can be added, replaced, attached, detached and removed.  In addition, log
     devices are imported and exported as part of the pool that contains them.  Mirrored devices
     can be removed by specifying the top-level mirror vdev.

   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 allow much more of this working set to be served from low latency media.  Using
     cache devices provides 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 1GB we do not write the metadata
     structures required for rebuilding the L2ARC in order not to waste space. This can be
     changed with l2arc_rebuild_blocks_min_l2size.  The cache device header (512 bytes) is
     updated 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 are not going to 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/online the cache device
     when there is less memory pressure in order to fully restore its contents to L2ARC.

   Pool checkpoint
     Before starting critical procedures that include destructive actions (e.g 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 checkpoint.  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 properties to vdev configuration.  Thus,
     while a pool has a checkpoint certain operations are not allowed.  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 checkpoint 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

     To discard the checkpoint from a pool:

     # zpool checkpoint -d pool

     Dataset reservations (controlled by the reservation or refreservation zfs 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
     The allocations in the special class are dedicated to specific block types.  By default this
     includes all metadata, the indirect blocks of user data, and any deduplication tables.  The
     class can also be provisioned to accept small file blocks.

     A pool must always have at least one normal (non-dedup/special) vdev before 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 setting the
     zfs_ddt_data_is_special zfs module parameter to false (0).

     Inclusion of small file blocks in the special class is opt-in. Each dataset can control the
     size of small file blocks allowed in the special class by setting the special_small_blocks
     dataset property. It defaults to zero, so you must opt-in by setting it to a non-zero value.
     See zfs(8) for more info on setting this property.