Reorganize zpool(8) man page into sections

Moved subcommand topics into individual manpages. Reordered and 
grouped the list of subcommands by topic.

Moved concepts overview to `zpoolconcepts.8` and the long list of
available pool properties to `zpoolprops.8`.

Internal cross-references copied from `zpool.8` needed to be 
converted to `.Xr` external references to new subcommand manual 
pages.

Move `autotrim` into lexical order, autotrim tacked onto the end
of a list. Now it is in alphabetical order.

Clarify attach/detach description. Description was too specific to
command syntax. Overview clarifies reason for attaching or detaching
a device.

Clarify replace description, don't refer to subcommand arguments.

Clarify split command description, say what split actually does and
why you'd want to do it.

Clarify description of upgrade, and simplify the zpool.8 wording of
the zpool-upgrade(8) description.

Clarify description of import, detail what zpool-import(8) actually 
does.

Add appropriate SEE ALSO sections. Divided zpool subcommand manual 
pages need their own SEE ALSO sections. Also modified fsck.zfs.8 
to point directly to zfs-scrub.8 and zed.8.in to include a direct
reference to zfs-events.8

Reviewed-by: Matt Ahrens <[email protected]>
Reviewed-by: Kjeld Schouten <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Ross Williams <[email protected]>
Closes #9564 

1 files changed, 394 insertions, 0 deletions

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+.\" Copyright (c) 2007, Sun Microsystems, Inc. All Rights Reserved.
+.\" Copyright (c) 2012, 2018 by Delphix. All rights reserved.
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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 considered volatile, as is the case with
+other system caches.
+.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.