FreeBSD Manual Pages
ZPOOLCONCEPTS(7) 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 de- vices, organized according to certain performance and fault character- istics. The following virtual devices are supported: disk A block device, typically located under /dev. ZFS can use in- dividual 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 auto- matically 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 experimen- tal purposes, as the fault tolerance of a file is only as good as the file system on which it resides. A file must be speci- fied 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 distributed-parity layout, similar to RAID-5/6, with im- proved distribution of parity, and which does not suffer from the RAID-5/6 "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, though not necessarily in a consistent stripe width. A raidz group can have single, double, or triple parity, mean- ing that the raidz group can sustain one, two, or three fail- ures, 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, allowing 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 chil- dren in order to fully utilize the available disk performance. Unlike raidz, dRAID uses a fixed stripe width (padding as nec- essary with zeros) to allow fully sequential resilvering. This fixed stripe width significantly affects both usable ca- pacity and IOPS. For example, with the default D=8 and 4 KiB disk sectors the minimum allocation size is 32 KiB. If using compression, this relatively large allocation size can reduce the effective compression ratio. When using ZFS volumes (zvols) 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 recom- mended 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 raidz, a dRAID can have single-, double-, or triple-par- ity. 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 de- vices failing without losing data. draid[parity][:datad][:childrenc][:sparess] A non-default dRAID configuration can be specified by append- ing 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 resil- vering at the expense of total usable capacity. De- faults 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 chil- dren 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 solely dedicated for deduplication tables. The re- dundancy of this device should match the redundancy of the other normal devices in the pool. If more than one dedup de- vice is specified, then allocations are load-balanced between those devices. special A device dedicated solely for allocating various kinds of in- ternal metadata, and optionally small file blocks. The redun- dancy 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 can- not be configured as a mirror or raidz group. For more infor- mation, see the "Cache Devices" section. Virtual devices cannot be nested arbitrarily. A mirror, raidz or draid virtual device can only be created with files or disks. Mirrors of mirrors or other such combinations are not allowed. A pool can have any number of virtual devices at the top of the config- uration (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, sepa- rated by whitespace. Keywords like mirror and raidz are used to dis- tinguish where a group ends and another begins. For example, the fol- lowing 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 auto- matically repairs bad data from a good copy, when corruption is de- tected. 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 nor- mally. A degraded pool is one in which one or more devices have failed, but the data is still available due to a redundant configura- tion. 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 follow- ing states: DEGRADED One or more top-level vdevs is in the degraded state because one or more component devices are offline. Sufficient repli- cas 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 or slow I/Os exceeds ac- ceptable levels and the device is degraded as an indica- tion 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 un- derlying 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 de- vice. 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 run- ning. 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 in- stances 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 mir- rored copy). In this case, ZFS reports the checksum error against the disks that contained damaged data. If a block is unable to be recon- structed (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 at- tempts to bring the device online automatically. Device attachment de- tection is hardware-dependent and might not be supported on all plat- forms. 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 re- placed. At this point, the hot spare becomes available again, if an- other device fails. If a pool has a shared spare that is currently being used, the pool cannot 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 de- tected, 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 ex- ample: # 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 de- vices. 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 de- vices 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 al- lows 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 num- ber 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 reis- sued to the original storage pool device, which might be part of a mir- rored or raidz configuration. The content of the cache devices is persistent across reboots and re- stored asynchronously when importing the pool in L2ARC (persistent L2ARC). This can be disabled by setting l2arc_rebuild_enabled=0. For cache devices smaller than 1 GiB, ZFS does not write the metadata structures required for rebuilding the L2ARC, to conserve space. This can be changed with l2arc_rebuild_blocks_min_l2size. The cache device header (512 B) 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 will not 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 de- vice are not fully restored in L2ARC. The user can off- and online the cache device when there is less memory pressure, 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 al- lowed while a pool has a checkpoint. Specifically, vdev removal/at- tach/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 unset- ting 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 spe- cial class by setting the special_small_blocks property to nonzero. See zfsprops(7) for more info on this property. FreeBSD Ports 14.quarterly April 7, 2023 ZPOOLCONCEPTS(7)
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=zpoolconcepts&sektion=7&manpath=FreeBSD+Ports+14.3.quarterly>