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doc: clarify distributed ephemeral pins new mechanism

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This documentation was for the old code, the new code (by Zheng)
fragments the directory and distributes those fragments.

Fixes: https://tracker.ceph.com/issues/51187
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
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Patrick Donnelly 2021-04-21 20:41:43 -07:00
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doc/cephfs/multimds.rst
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@ -164,10 +164,12 @@ your metadata throughput with no other administrative intervention.
Presently, there are two types of ephemeral pinning:
**Distributed Ephemeral Pins**: This policy indicates that **all** of a
directory's immediate children should be ephemerally pinned. The canonical
example would be the ``/home`` directory: we want every user's home directory
to be spread across the entire MDS cluster. This can be set via:
**Distributed Ephemeral Pins**: This policy causes a directory to fragment
(even well below the normal fragmentation thresholds) and distribute its
fragments as ephemerally pinned subtrees. This has the effect of distributing
immediate children across a range of MDS ranks. The canonical example use-case
would be the ``/home`` directory: we want every user's home directory to be
spread across the entire MDS cluster. This can be set via:
::
@ -221,18 +223,3 @@ For the reverse situation:
The ``home/patrick`` directory and its children will be pinned to rank 2
because its export pin overrides the policy on ``home``.
If a directory has an export pin and an ephemeral pin policy, the export pin
applies to the directory itself and the policy to its children. So:
::
mkdir -p home/{patrick,john}
setfattr -n ceph.dir.pin -v 0 home
setfattr -n ceph.dir.pin.distributed -v 1 home
The home directory inode (and all of its directory fragments) will always be
located on rank 0. All children including ``home/patrick`` and ``home/john``
will be ephemerally pinned according to the distributed policy. This may only
matter for some obscure performance advantages. All the same, it's mentioned
here so the override policy is clear.