||FreeBSD Miscellaneous Information Manual
overview of ZFS storage pools
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:
- 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.
- 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.
- 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,
- 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
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,
- 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
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
A dRAID with N disks of
size X, D
data disks per redundancy group,
P parity level, and
S distributed hot spares can hold
bytes and can withstand P
devices failing without losing data.
- A non-default dRAID configuration can be specified by appending one or
more of the following optional arguments to the draid
- The parity level (1-3).
- 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,
is less than 8.
- 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.
- The number of distributed hot spares. Defaults to zero.
- A pseudo-vdev which keeps track of available hot spares for a pool. For
more information, see the Hot Spares
- 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
- 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.
- 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
- 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
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:
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.
mirror sda sdb
mirror sdc sdd
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:
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
- One or more top-level vdevs is in the degraded state because one or more
component devices are offline. Sufficient replicas exist to continue
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
- One or more top-level vdevs is in the faulted state because one or more
component devices are offline. Insufficient replicas exist to continue
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
- The number of I/O errors exceeds acceptable levels and the device is
faulted to prevent further use of the device.
- The device was explicitly taken offline by the
- The device is online and functioning.
- The device was physically removed while the system was running. Device
removal detection is hardware-dependent and may not be supported on all
- 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
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
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.
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
mirror sda sdb spare
Spares can be shared across multiple pools, and can be added with
add command and
removed with the
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
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
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 cannot replace log devices.
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
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
create pool sda sdb
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.
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
To create a pool with cache devices, specify a
cache vdev with any number of devices. For example:
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
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
Before starting critical procedures that include destructive actions (like
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
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.
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:
To later rewind to its checkpointed state, you need to first
export it and then rewind it during import:
To discard the checkpoint from a pool:
Dataset reservations (controlled by the
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
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
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
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
for more info on this property.
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