pveceph: add section about erasure code pools

Signed-off-by: Aaron Lauterer <a.lauterer@proxmox.com>

diff --git a/pveceph.adoc b/pveceph.adoc
index 0984a52b294b76e11d283548c8c7b1c301db4c45..dff44e03345e8d0c8a2dd566e1ed238fcdb8280e 100644 (file)
--- a/pveceph.adoc
+++ b/pveceph.adoc
@@ -547,6 +547,71 @@ operation footnote:[Ceph pool operation
 manual.
 
 
+[[pve_ceph_ec_pools]]
+Erasure Coded (EC) Pools
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Erasure coded (EC) pools can offer more usable space for the price of
+performance. In replicated pools, multiple replicas of the data are stored
+('size'). In erasure coded pool, data is split into 'k' data chunks with
+additional 'm' coding chunks.  The coding chunks can be used to recreate data
+should data chunks be missing.  The number of coding chunks, 'm', defines how
+many OSDs can be lost without losing any data.  The total amount of objects
+stored is 'k + m'.
+
+The default 'min_size' of an EC pool depends on the 'm' parameter. If 'm = 1',
+the 'min_size' of the EC pool will be 'k'. The 'min_size' will be 'k + 1' if
+'m > 1'. The Ceph documentation recommends a conservative 'min_size' of 'k + 2'
+footnote:[Ceph Erasure Coded Pool Recovery
+{cephdocs-url}/rados/operations/erasure-code/#erasure-coded-pool-recovery].
+
+If there are less than 'min_size' OSDs available, any IO to the pool will be
+blocked until there are enough OSDs available again.
+
+NOTE: When planning an erasure coded pool, keep an eye on the 'min_size' as it
+defines how many OSDs need to be available. Otherwise, IO will be blocked.
+
+For example, an EC pool with 'k = 2' and 'm = 1' will have 'size = 3',
+'min_size = 2' and will stay operational if one OSD fails. If the pool is
+configured with 'k = 2', 'm = 2', it will have a 'size = 4' and 'min_size = 3'
+and stay operational if one OSD is lost.
+
+To create a new EC pool, run the following command:
+
+[source,bash]
+----
+pceveph pool create <pool name> --erasure-coding k=2,m=1
+----
+
+Optional parameters are 'failure-domain' and 'device-class'. If you
+need to change any EC profile settings used by the pool, you will have to
+create a new pool with a new profile.
+
+This will create a new EC pool plus the needed replicated pool to store the RBD
+omap and other metadata. In the end, there will be a '<pool name>-data' and
+'<pool name>-metada' pool. The default behavior is to create a matching storage
+configuration as well. If that behavior is not wanted, you can disable it by
+providing the '--add_storages 0' parameter.  When configuring the storage
+configuration manually, keep in mind that the 'data-pool' parameter needs to be
+set. Only then will the EC pool be used to store the data objects. For example:
+
+[source,bash]
+----
+pvesm add rbd <storage name> --pool <replicated pool> --data-pool <ec pool>
+----
+
+If there is a need to further customize the EC profile, you can do so by
+creating it with the Ceph tools directly footnote:[Ceph Erasure Code Profile
+{cephdocs-url}/rados/operations/erasure-code/#erasure-code-profiles], and
+specify the profile to use with the 'profile' parameter.
+
+For example:
+[source,bash]
+----
+pceveph pool create <pool name> --erasure-coding profile=<profile name>
+----
+
+
 Destroy Pools
 ~~~~~~~~~~~~~