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fix #3967: add ZFS dRAID documentation

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add some basic explanation how ZFS dRAID works including
links to openZFS for more details

add documentation for two dRAID parameters used in code

Signed-off-by: Stefan Hrdlicka <s.hrdlicka@proxmox.com>
This commit is contained in:
Stefan Hrdlicka 2022-11-10 14:24:50 +01:00 committed by Thomas Lamprecht
parent 4f138470fc
commit 447596fd16
1 changed files with 43 additions and 1 deletions
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local-zfs.adoc
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@ -32,7 +32,8 @@ management.
* Copy-on-write clone
* Various raid levels: RAID0, RAID1, RAID10, RAIDZ-1, RAIDZ-2 and RAIDZ-3
* Various raid levels: RAID0, RAID1, RAID10, RAIDZ-1, RAIDZ-2, RAIDZ-3,
dRAID, dRAID2, dRAID3
* Can use SSD for cache
@ -244,6 +245,47 @@ them, unless your environment has specific needs and characteristics where
RAIDZ performance characteristics are acceptable.
ZFS dRAID
~~~~~~~~~
In a ZFS dRAID (declustered RAID) the hot spare drive(s) participate in the RAID.
Their spare capacity is reserved and used for rebuilding when one drive fails.
This provides, depending on the configuration, faster rebuilding compared to a
RAIDZ in case of drive failure. More information can be found in the official
OpenZFS documentation. footnote:[OpenZFS dRAID
https://openzfs.github.io/openzfs-docs/Basic%20Concepts/dRAID%20Howto.html]
NOTE: dRAID is intended for more than 10-15 disks in a dRAID. A RAIDZ
setup should be better for a lower amount of disks in most use cases.
NOTE: The GUI requires one more disk than the minimum (i.e. dRAID1 needs 3). It
expects that a spare disk is added as well.
* `dRAID1` or `dRAID`: requires at least 2 disks, one can fail before data is
lost
* `dRAID2`: requires at least 3 disks, two can fail before data is lost
* `dRAID3`: requires at least 4 disks, three can fail before data is lost
Additional information can be found on the manual page:
----
# man zpoolconcepts
----
Spares and Data
^^^^^^^^^^^^^^^
The number of `spares` tells the system how many disks it should keep ready in
case of a disk failure. The default value is 0 `spares`. Without spares,
rebuilding won't get any speed benefits.
`data` defines the number of devices in a redundancy group. The default value is
8. Except when `disks - parity - spares` equal something less than 8, the lower
number is used. In general, a smaller number of `data` devices leads to higher
IOPS, better compression ratios and faster resilvering, but defining fewer data
devices reduces the available storage capacity of the pool.
Bootloader
~~~~~~~~~~