ZPOOLCONCEPTS(7) FreeBSD 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
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.
FreeBSD 13.1-RELEASE-p6 June 2, 2021 FreeBSD 13.1-RELEASE-p6
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