zfs/man/man8/zpoolconcepts.8

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.Dd August 9, 2019
.Dt ZPOOLCONCEPTS 8
.Os Linux
.Sh NAME
.Nm zpoolconcepts
.Nd overview of ZFS storage pools
.Sh DESCRIPTION
.Ss 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:
.Bl -tag -width Ds
.It Sy disk
A block device, typically located under
.Pa /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
.Po the relative portion of the path under
.Pa /dev
.Pc .
A whole disk can be specified by omitting the slice or partition designation.
For example,
.Pa sda
is equivalent to
.Pa /dev/sda .
When given a whole disk, ZFS automatically labels the disk, if necessary.
.It Sy 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.
.It Sy 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.
.It Sy raidz , raidz1 , raidz2 , raidz3
A variation on RAID-5 that allows for better distribution of parity and
eliminates the RAID-5
.Qq write hole
.Pq in which data and parity become inconsistent after a power loss .
Data and parity is striped across all disks within a raidz group.
.Pp
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
.Sy raidz1
vdev type specifies a single-parity raidz group; the
.Sy raidz2
vdev type specifies a double-parity raidz group; and the
.Sy raidz3
vdev type specifies a triple-parity raidz group.
The
.Sy raidz
vdev type is an alias for
.Sy raidz1 .
.Pp
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.
.It Sy spare
A pseudo-vdev which keeps track of available hot spares for a pool.
For more information, see the
.Sx Hot Spares
section.
.It Sy 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
.Sx Intent Log
section.
.It Sy 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.
.It Sy 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.
.Pp
For more information on special allocations, see the
.Sx Special Allocation Class
section.
.It Sy 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
.Sx Cache Devices
section.
.El
.Pp
Virtual devices cannot be nested, so a mirror or raidz virtual device can only
contain files or disks.
Mirrors of mirrors
.Pq or other combinations
are not allowed.
.Pp
A pool can have any number of virtual devices at the top of the configuration
.Po known as
.Qq root vdevs
.Pc .
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.
.Pp
Virtual devices are specified one at a time on the command line, separated by
whitespace.
The keywords
.Sy mirror
and
.Sy 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:
.Bd -literal
# zpool create mypool mirror sda sdb mirror sdc sdd
.Ed
.Ss 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.
.Pp
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.
.Pp
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.
.Pp
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:
.Bl -tag -width "DEGRADED"
.It Sy 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.
.Pp
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:
.Bl -bullet
.It
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.
.It
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.
.El
.It Sy 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.
.Pp
One or more component devices is in the faulted state, and insufficient
replicas exist to continue functioning.
The underlying conditions are as follows:
.Bl -bullet
.It
The device could be opened, but the contents did not match expected values.
.It
The number of I/O errors exceeds acceptable levels and the device is faulted to
prevent further use of the device.
.El
.It Sy OFFLINE
The device was explicitly taken offline by the
.Nm zpool Cm offline
command.
.It Sy ONLINE
The device is online and functioning.
.It Sy 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.
.It Sy 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.
.El
.Pp
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.
.Ss Hot Spares
ZFS allows devices to be associated with pools as
.Qq 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
.Sy spare
vdev with any number of devices.
For example,
.Bd -literal
# zpool create pool mirror sda sdb spare sdc sdd
.Ed
.Pp
Spares can be shared across multiple pools, and can be added with the
.Nm zpool Cm add
command and removed with the
.Nm zpool Cm remove
command.
Once a spare replacement is initiated, a new
.Sy 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.
.Pp
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.
.Pp
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.
.Pp
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.
.Pp
Spares cannot replace log devices.
.Ss 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
.Xr 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:
.Bd -literal
# zpool create pool sda sdb log sdc
.Ed
.Pp
Multiple log devices can also be specified, and they can be mirrored.
See the
.Sx EXAMPLES
section for an example of mirroring multiple log devices.
.Pp
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.
.Ss Cache Devices
Devices can be added to a storage pool as
.Qq 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.
.Pp
To create a pool with cache devices, specify a
.Sy cache
vdev with any number of devices.
For example:
.Bd -literal
# zpool create pool sda sdb cache sdc sdd
.Ed
.Pp
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.
.Pp
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
.Sy 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
.Sy l2arc_rebuild_blocks_min_l2size .
The cache device header (512 bytes) is updated even if no metadata structures
are written. Setting
.Sy 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
.Nm zpool Cm 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
.Nm zpool Cm 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.
.Ss Pool checkpoint
Before starting critical procedures that include destructive actions (e.g
.Nm zfs Cm 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.
.Pp
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.
.Pp
To create a checkpoint for a pool:
.Bd -literal
# zpool checkpoint pool
.Ed
.Pp
To later rewind to its checkpointed state, you need to first export it and
then rewind it during import:
.Bd -literal
# zpool export pool
# zpool import --rewind-to-checkpoint pool
.Ed
.Pp
To discard the checkpoint from a pool:
.Bd -literal
# zpool checkpoint -d pool
.Ed
.Pp
Dataset reservations (controlled by the
.Nm reservation
or
.Nm 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.
.Ss 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.
.Pp
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.
.Pp
Deduplication tables can be excluded from the special class by setting the
.Sy zfs_ddt_data_is_special
zfs module parameter to false (0).
.Pp
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
.Sy special_small_blocks
dataset property. It defaults to zero, so you must opt-in by setting it to a
non-zero value. See
.Xr zfs 8
for more info on setting this property.