[pve-devel] [PATCH pve-docs 3/3] fix #3967: add ZFS dRAID documentation

[Stefan Hrdlicka <s.hrdlicka@proxmox.com>]

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>
---
 local-zfs.adoc | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/local-zfs.adoc b/local-zfs.adoc
index ab0f6ad..8eb681c 100644
--- a/local-zfs.adoc
+++ b/local-zfs.adoc
@@ -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 +245,43 @@ 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 reservered and used for rebuilding when one drive fails.
+This provides depending on the configuration faster rebuilding compaired to a
+RAIDZ in case of drive failure. More information can be found in the official
+openZFS documenation. footnote:[OpenZFS dRAID
+https://openzfs.github.io/openzfs-docs/Basic%20Concepts/dRAID%20Howto.html]
+
+NOTE: dRAID is intended for more then 10-15 disks in a dRAID. A RAIDZ
+setup should be better for a lower amount of disks in most use cases.
+
+ * `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
+
+
+Additonal information can be found on manual page:
+
+----
+# man zpoolconcepts
+----
+
+spares and data
+^^^^^^^^^^^^^^^
+The number of `spares` tells the system how many disks it should keep ready in
+case of of a disk failure. The default value is 0 `spares`. Without spares
+rebuilding won't get any speed benefits.
+
+The number of `data` devices specifies the size of a parity group. The default
+is 8 if the number of `disks - parity - spares >= 8`. A higher number of `data`
+and parity drives increases the allocation size (e.g. for 4k sectors with
+default `data`=6 minimum allocation size is 24k) which can affect compression.
+
+
 Bootloader
 ~~~~~~~~~~
 
-- 
2.30.2