|ZPOOLPROPS(7)||Miscellaneous Information Manual||ZPOOLPROPS(7)|
properties of ZFS storage pools
Each pool has several properties associated with it. Some properties are read-only statistics while others are configurable and change the behavior of the pool.
The following are read-only properties:
- Amount of storage used within the pool. See fragmentation and free for more information.
- Percentage of pool space used. This property can also be referred to by its shortened column name, cap.
- Amount of uninitialized space within the pool or device that can be used
to increase the total capacity of the pool. On whole-disk vdevs, this is
the space beyond the end of the GPT – typically occurring when a
LUN is dynamically expanded or a disk replaced with a larger one. On
partition vdevs, this is the space appended to the partition after it was
added to the pool – most likely by resizing it in-place. The space
can be claimed for the pool by bringing it online with
- The amount of fragmentation in the pool. As the amount of space allocated increases, it becomes more difficult to locate free space. This may result in lower write performance compared to pools with more unfragmented free space.
- The amount of free space available in the pool. By contrast, the zfs(8) available property describes how much new data can be written to ZFS filesystems/volumes. The zpool free property is not generally useful for this purpose, and can be substantially more than the zfs available space. This discrepancy is due to several factors, including raidz parity; zfs reservation, quota, refreservation, and refquota properties; and space set aside by spa_slop_shift (see zfs(4) for more information).
- After a file system or snapshot is destroyed, the space it was using is returned to the pool asynchronously. freeing is the amount of space remaining to be reclaimed. Over time freeing will decrease while free increases.
- Space not released while freeing due to corruption, now permanently leaked into the pool.
- The current health of the pool. Health can be one of ONLINE, DEGRADED, FAULTED, OFFLINE, REMOVED, UNAVAIL.
- A unique identifier for the pool.
- A unique identifier for the pool. Unlike the guid property, this identifier is generated every time we load the pool (i.e. does not persist across imports/exports) and never changes while the pool is loaded (even if a reguid operation takes place).
- Total size of the storage pool.
- Information about unsupported features that are enabled on the pool. See zpool-features(7) for details.
The space usage properties report actual physical space available
to the storage pool. The physical space can be different from the total
amount of space that any contained datasets can actually use. The amount of
space used in a raidz configuration depends on the characteristics of the
data being written. In addition, ZFS reserves some space for internal
accounting that the zfs(8) command takes into account, but
zpoolprops command does not. For non-full pools
of a reasonable size, these effects should be invisible. For small pools, or
pools that are close to being completely full, these discrepancies may
become more noticeable.
The following property can be set at creation time and import time:
- Alternate root directory. If set, this directory is prepended to any mount points within the pool. This can be used when examining an unknown pool where the mount points cannot be trusted, or in an alternate boot environment, where the typical paths are not valid. altroot is not a persistent property. It is valid only while the system is up. Setting altroot defaults to using cachefile=none, though this may be overridden using an explicit setting.
The following property can be set only at import time:
- If set to on, the pool will be imported in read-only mode. This property can also be referred to by its shortened column name, rdonly.
The following properties can be set at creation time and import
time, and later changed with the
- Pool sector size exponent, to the power of 2 (internally referred to as ashift). Values from 9 to 16, inclusive, are valid; also, the value 0 (the default) means to auto-detect using the kernel's block layer and a ZFS internal exception list. I/O operations will be aligned to the specified size boundaries. Additionally, the minimum (disk) write size will be set to the specified size, so this represents a space vs. performance trade-off. For optimal performance, the pool sector size should be greater than or equal to the sector size of the underlying disks. The typical case for setting this property is when performance is important and the underlying disks use 4KiB sectors but report 512B sectors to the OS (for compatibility reasons); in that case, set ashift=12 (which is 1<<12 = 4096). When set, this property is used as the default hint value in subsequent vdev operations (add, attach and replace). Changing this value will not modify any existing vdev, not even on disk replacement; however it can be used, for instance, to replace a dying 512B sectors disk with a newer 4KiB sectors device: this will probably result in bad performance but at the same time could prevent loss of data.
- Controls automatic pool expansion when the underlying LUN is grown. If set to on, the pool will be resized according to the size of the expanded device. If the device is part of a mirror or raidz then all devices within that mirror/raidz group must be expanded before the new space is made available to the pool. The default behavior is off. This property can also be referred to by its shortened column name, expand.
- Controls automatic device replacement. If set to off,
device replacement must be initiated by the administrator by using the
replacecommand. If set to on, any new device, found in the same physical location as a device that previously belonged to the pool, is automatically formatted and replaced. The default behavior is off. This property can also be referred to by its shortened column name, replace. Autoreplace can also be used with virtual disks (like device mapper) provided that you use the /dev/disk/by-vdev paths setup by vdev_id.conf. See the vdev_id(8) manual page for more details. Autoreplace and autoonline require the ZFS Event Daemon be configured and running. See the zed(8) manual page for more details.
- When set to on space which has been recently freed, and
is no longer allocated by the pool, will be periodically trimmed. This
allows block device vdevs which support BLKDISCARD, such as SSDs, or file
vdevs on which the underlying file system supports hole-punching, to
reclaim unused blocks. The default value for this property is
Automatic TRIM does not immediately reclaim blocks after a free. Instead, it will optimistically delay allowing smaller ranges to be aggregated into a few larger ones. These can then be issued more efficiently to the storage. TRIM on L2ARC devices is enabled by setting l2arc_trim_ahead > 0.
Be aware that automatic trimming of recently freed data blocks can put significant stress on the underlying storage devices. This will vary depending of how well the specific device handles these commands. For lower-end devices it is often possible to achieve most of the benefits of automatic trimming by running an on-demand (manual) TRIM periodically using the
- Identifies the default bootable dataset for the root pool. This property is expected to be set mainly by the installation and upgrade programs. Not all Linux distribution boot processes use the bootfs property.
- Controls the location of where the pool configuration is cached.
Discovering all pools on system startup requires a cached copy of the
configuration data that is stored on the root file system. All pools in
this cache are automatically imported when the system boots. Some
environments, such as install and clustering, need to cache this
information in a different location so that pools are not automatically
imported. Setting this property caches the pool configuration in a
different location that can later be imported with
-c. Setting it to the value none creates a temporary pool that is never cached, and the "" (empty string) uses the default location.
Multiple pools can share the same cache file. Because the kernel destroys and recreates this file when pools are added and removed, care should be taken when attempting to access this file. When the last pool using a cachefile is exported or destroyed, the file will be empty.
- A text string consisting of printable ASCII characters that will be stored such that it is available even if the pool becomes faulted. An administrator can provide additional information about a pool using this property.
- Specifies that the pool maintain compatibility with specific feature sets. When set to off (or unset) compatibility is disabled (all features may be enabled); when set to legacyno features may be enabled. When set to a comma-separated list of filenames (each filename may either be an absolute path, or relative to /etc/zfs/compatibility.d or /usr/share/zfs/compatibility.d) the lists of requested features are read from those files, separated by whitespace and/or commas. Only features present in all files may be enabled.
- This property is deprecated and no longer has any effect.
- Controls whether a non-privileged user is granted access based on the dataset permissions defined on the dataset. See zfs(8) for more information on ZFS delegated administration.
- Controls the system behavior in the event of catastrophic pool failure.
This condition is typically a result of a loss of connectivity to the
underlying storage device(s) or a failure of all devices within the pool.
The behavior of such an event is determined as follows:
- Blocks all I/O access until the device connectivity is recovered and
the errors are cleared with
clear. This is the default behavior.
EIOto any new write I/O requests but allows reads to any of the remaining healthy devices. Any write requests that have yet to be committed to disk would be blocked.
- Prints out a message to the console and generates a system crash dump.
- The value of this property is the current state of feature_name. The only valid value when setting this property is enabled which moves feature_name to the enabled state. See zpool-features(7) for details on feature states.
- Controls whether information about snapshots associated with this pool is
listis run without the
-toption. The default value is off. This property can also be referred to by its shortened name, listsnaps.
- Controls whether a pool activity check should be performed during
import. When a pool is determined to be active it cannot be imported, even with the
-foption. This property is intended to be used in failover configurations where multiple hosts have access to a pool on shared storage.
Multihost provides protection on import only. It does not protect against an individual device being used in multiple pools, regardless of the type of vdev. See the discussion under
When this property is on, periodic writes to storage occur to show the pool is in use. See zfs_multihost_interval in the zfs(4) manual page. In order to enable this property each host must set a unique hostid. See genhostid(1) zgenhostid(8) spl(4) for additional details. The default value is off.
- The current on-disk version of the pool. This can be increased, but never
decreased. The preferred method of updating pools is with the
upgradecommand, though this property can be used when a specific version is needed for backwards compatibility. Once feature flags are enabled on a pool this property will no longer have a value.
|May 27, 2021||OpenZFS|