[pve-devel] [PATCH pve-docs] Add section for ZFS Special Device

[Fabian Ebner]

Signed-off-by: Fabian Ebner <f.ebner at proxmox.com>
---
 local-zfs.adoc | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/local-zfs.adoc b/local-zfs.adoc
index b4fb7db..378cbee 100644
--- a/local-zfs.adoc
+++ b/local-zfs.adoc
@@ -431,3 +431,47 @@ See the `encryptionroot`, `encryption`, `keylocation`, `keyformat` and
 `keystatus` properties, the `zfs load-key`, `zfs unload-key` and `zfs
 change-key` commands and the `Encryption` section from `man zfs` for more
 details and advanced usage.
+
+
+ZFS Special Device
+~~~~~~~~~~~~~~~~~~
+
+Since version 0.8.0 ZFS allows adding a `special` device to a pool, which is
+then used to store metadata, deduplication tables and optionally small file
+blocks.
+
+IMPORTANT: The redundancy of the `special` device should match the one of the
+pool, since the `special` device is a point of failure for the whole pool.
+
+WARNING: Adding a `special` device to a pool cannot be undone!
+
+.Create a pool with `special` device and RAID-1:
+
+ zpool create -f -o ashift=12 <pool> mirror <device1> <device2> special mirror <device3> <device4>
+
+.Add a `special` device to an existing pool with RAID-1:
+
+ zpool add <pool> special mirror <device1> <device2>
+
+For ZFS datasets where the `special_small_blocks` property is set to a non-zero
+value, the `special` device is used to store small file blocks up to that size.
+Setting the `special_small_blocks` property on the pool will change the default
+value of that property for all child ZFS datasets (for example all containers
+in the pool will opt in for small file blocks).
+
+.Opt in for small file blocks pool-wide:
+
+ zfs set special_small_blocks=<size> <pool>
+
+.Opt in for small file blocks for a single dataset:
+
+ zfs set special_small_blocks=<size> <pool>/<filesystem>
+
+.Opt out from small file blocks for a single dataset:
+
+ zfs set special_small_blocks=0 <pool>/<filesystem>
+
+Using a `special` device makes sense for pools with lots and lots of changing
+metadata respectively small files. If you also have other, larger I/O on the
+same pool then the benefit from using a `special` device might be even more
+noticeable. It is recommended to use SSDs or NVMes for the `special` device.
-- 
2.20.1