Provided by: zfsutils-linux_2.1.5-1ubuntu6_amd64 
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 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 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 without losing data.
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 devices failing without losing data. 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.
draid, draid1, draid2, draid3
A variant of raidz that provides integrated distributed hot spares which allows for
faster resilvering while retaining the benefits 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 necessary with zeros) to
allow fully sequential resilvering. This fixed stripe width significantly effects
both usable capacity and IOPS. For example, with the default D=8 and 4kB disk
sectors the minimum allocation size is 32kB. If using compression, this relatively
large allocation size can reduce the effective compression ratio. When using ZFS
volumes and dRAID, the default of the volblocksize property is increased to account
for the allocation 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 raidzm 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 keyword:
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 differs.
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 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.
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 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 a 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.
Checksum errors represent events where a disk returned data that was expected 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 damaged 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 online the device
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 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.
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 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 allows 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 (512B) 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- and 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 (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 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,
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 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 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, 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 unsetting the
zfs_ddt_data_is_special ZFS module parameter.
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
property to nonzero. See zfsprops(7) for more info on this property.