On 3/24/20 8:19 AM, Maged Mokhtar wrote:
On 24/03/2020 13:35, Daniel Gryniewicz wrote:
On 3/23/20 4:31 PM, Maged Mokhtar wrote:
On 23/03/2020 20:50, Jeff Layton wrote:
On Mon, 2020-03-23 at 15:49 +0200, Maged Mokhtar wrote:
Hello all,
For multi-node NFS Ganesha over CephFS, is it OK to leave libcephfs
write caching on, or should it be configured off for failover ?
You can do libcephfs write caching, as the caps would need to be
recalled for any competing access. What you really want to avoid is any
sort of caching at the ganesha daemon layer.
Hi Jeff,
Thanks for your reply. I meant caching by libcepfs used within the
ganesha ceph fsal plugin, which i am not sure from your reply if this
is what you refer to as ganesha daemon layer (or does the later mean
the internal mdcache in ganesha). I really appreciate if you can
clarify this point.
Caching in libcephfs is fine, it's caching above the FSAL layer that
you should avoid.
I really have doubts that it is safe to leave write caching in the
plugin and have safe failover, yet i see comments in the conf file
such as:
# The libcephfs client will aggressively cache information while it
# can, so there is little benefit to ganesha actively caching the same
# objects.
Or is it up to the NFS client to issue cache syncs and re-submit
writes if it detects failover ?
Correct. During failover, NFS will go into it's Grace period, which
blocks new state, and allow the NFS clients to re-acquire the state
(opens, locks, delegations, etc.). This includes re-sending any
non-committed writes (commits will cause the data to be saved to the
cluster, not just the libcephfs cache). Once this is all done, normal
operation proceeds. It should be safe, even with caching in libcephfs.
Daniel
Thanks Daniel for the clarification..so it is the responsibility of the
client tor re-send writes...2 questions so i can understand this better:
-If this is handled at the client..why on the gateway it is ok to cache
at the FSAL layer but not above ?
In principle, it's fine above. However, that requires a level of
coordination that's not there right now. The libcephfs cache is
integrated with the CAPs system, and knows when it can cache and when it
needs to flush. There's work to do to get that up to the higher layers.
-At what level/layer on the client does this get handled: NFS client
layer (which will detect failover), filesystem layer, page cache...?
The NFS client layer, interacting with the VFS/page cache. (NFS is the
filesystem in this case, so technically the filesystem layer.)
Daniel
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