ZPOOLCONCEPTS(7)

zpoolconcepts — aka the ZFS storage pool tea ☕✨

Section 7, Miscellaneous Information Manual 💖 bestie, pull up a chair, we're talking storage pools and it's gonna be a lot.

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✨ NAME ✨

zpoolconcepts — the whole vibe of ZFS storage pools, explained 💅

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🌸 DESCRIPTION 🌸

🦋 Virtual Devices (vdevs) 🦋

okay so a "virtual device" is like... one single device, OR a whole lil friend group of devices, organized around how fast they are and how likely they are to betray you (performance and fault characteristics, bestie). here's the roster of what's supported:

• disk — a block device, usually chilling under /dev. ZFS can use individual slices or partitions, but no cap, the recommended mode is whole disks. You can point at a disk with a full path, or just use the shorthand name (the part of the path that lives under /dev). To specify a whole disk, just omit the slice or partition designation. Like, sda is literally equivalent to /dev/sda. And when you hand it a whole disk, ZFS automatically labels the disk if it needs to. iconic behavior honestly. 🫶
• file — a regular file. bestie. strongly discouraged, I'm not kidding. This exists primarily for experimental purposes, because a file's fault tolerance is only as good as the file system it's sitting on. 😭 A file must be specified by a full path. no shorthand, no shortcuts.
• mirror — a mirror of two or more devices. Data gets replicated in an identical fashion across every component of the mirror. A mirror with N disks of size X can hold X bytes and can withstand N-1 devices failing without losing data. it's giving ✨redundancy✨.
• raidz, raidz1, raidz2, raidz3 — a distributed-parity layout, similar to RAID-5/6 but with better parity distribution, and — huge W — it does not suffer from the RAID-5/6 "write hole" (that cursed situation where data and parity become inconsistent after a power loss 💀). Data and parity get 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, meaning it can sustain one, two, or three failures respectively without losing any data. So: raidz1 = single-parity raidz group, raidz2 = double-parity, raidz3 = triple-parity. And heads up, 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 has integrated distributed hot spares, which means faster resilvering while still keeping all the raidz perks. A dRAID vdev is built from multiple internal raidz groups, each with D data devices and P parity devices. These groups are distributed over all of the children so you can fully utilize the available disk performance. slay.

Unlike raidz, dRAID uses a fixed stripe width (padding with zeros as necessary) so it can do fully sequential resilvering. That fixed stripe width significantly affects both usable capacity AND IOPS, so pay attention. For example: with the default D=8 and 4 KiB disk sectors, the minimum allocation size is 32 KiB. If you're using compression, that relatively large allocation size can reduce the effective compression ratio. And when you're using ZFS volumes (zvols) with dRAID, the default of the volblocksize property gets increased to account for the allocation size. If a dRAID pool is gonna hold a significant amount of small blocks, the move is to also add a mirrored special vdev to store those blocks.

As far as I/O goes, 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-parity. The draid1, draid2, and draid3 types specify the parity level. And 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 devices failing without losing data. 💪

• draid[parity][:datad][:childrenc][:sparess] — you can specify a non-default dRAID configuration by appending one or more of these 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 resilvering, at the expense of total usable capacity. Defaults to 8, unless N-P-S is less than 8.
  • children — the expected number of children. Useful as a cross-check when you're listing a large number of devices. An error is returned when the provided number of children differs. 🚨
  • spares — the number of distributed hot spares. Defaults to zero.
• spare — a pseudo-vdev that keeps track of the available hot spares for a pool. For more info, see the "Hot Spares" section. 🧚
• log — a separate intent log device. If you specify more than one log device, writes get load-balanced between them. Log devices can be mirrored. BUT — raidz vdev types are not supported for the intent log, periodt. For more info, see the "Intent Log" section.
• dedup — a device solely dedicated to deduplication tables. The redundancy of this device should match the redundancy of the other normal devices in the pool. If you specify more than one dedup device, allocations get load-balanced between them.
• special — a device dedicated solely to allocating various kinds of internal metadata, and optionally small file blocks. Same deal: its redundancy should match the redundancy of the other normal devices in the pool. If you specify more than one special device, allocations get load-balanced between them.

For more info on special allocations, see the "Special Allocation Class" section. 💜

• cache — a device used to cache storage pool data. A cache device cannot be configured as a mirror or raidz group. For more info, see the "Cache Devices" section.

🚨 vibe check: 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? Not allowed. no exceptions bestie.

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A pool can have any number of virtual devices at the top of the configuration (these are the "root vdevs"). Data is dynamically distributed across all top-level devices to balance data among them. As new virtual devices get added, ZFS automatically places data on the newly available devices. main character energy fr. 👑

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, this creates a pool with two root vdevs, each a mirror of two disks:

# zpool create mypool mirror sda sdb mirror sdc sdd

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💀 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. it literally heals itself, iykyk ✨

But — to actually take advantage of all this, a pool must make use of some form of redundancy, using either mirrored or raidz groups. ZFS does support running in a non-redundant configuration, where each root vdev is simply a disk or file, but this is strongly discouraged. A single case of bit corruption can render some or all of your data unavailable. 😭 do not do this to yourself.

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 where one or more devices have failed, but the data is still available thanks 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 a top-level vdev (like 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 these states:

• 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.

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 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 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 underlying 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 device.

• OFFLINE — the device was explicitly taken offline by the zpool offline command. 🙈
• ONLINE — the device is online and functioning. 💅
• REMOVED — bestie, the device got physically yanked out while the system was still running 😭. Detecting that a device got removed is hardware-dependent, so it might not be supported on all platforms. iykyk.
• UNAVAIL — the device could not be opened, periodt. And heads up: if a pool is imported while a device was unavailable, then that device gets identified by a unique identifier instead of its path — because the path was never correct in the first place. 💀

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Checksum errors are what happens when a disk returns data that was supposed to be correct but wasn't 😳. In other words, these are instances of silent data corruption. The checksum errors get 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 mirrored copy) ✨. In that 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 that 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 to bring the device online automatically 🫶. Device attachment detection is hardware-dependent and might not be supported on all platforms.

💖 Hot Spares 💖

ZFS lets you associate devices with pools as "hot spares". These devices are not actively used in the pool. But — and this is the slay part — when an active device fails, it gets 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 replaced. At this point, the hot spare becomes available again, if another 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. no cap, that's real.

Shared spares add some risk, bestie 😭. 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 canceled 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. periodt.

🌸 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 example:

# 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 devices.

🎀 Cache Devices 🎀

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 static content.

To create a pool with cache devices, specify a cache vdev with any number 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 reissued to the original storage pool device, which might be part of a mirrored or raidz configuration 🔁.

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 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 device 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 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:

# 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

Note that rewinding to a checkpoint will permanently discard it. Once the pool has been successfully imported with the above rewind command, you cannot rewind to the same checkpoint 😭.

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, intent log (in absence of separate log device), and deduplication tables. The class can also be provisioned to accept small file blocks or zvol blocks on a per dataset granularity.

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 parameter.

Inclusion of small file or zvol 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 zfsprops(7) for more info on this property.

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and that's the whole vibe check, bestie 💖✨ this page is from the OpenZFS crew, dated August 6, 2025, and lives at ZPOOLCONCEPTS(7) — go forth and pool responsibly, mwah 💅🎀🦋