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 ? -At what level/layer on the client does this get handled: NFS client layer (which will detect failover), filesystem layer, page cache...? Thanks for your patience :) /Maged