Hi all,
Thanks for the interesting discussion. Actually it's a bit disappointing
to see that also cephfs with multiple MDS servers is not as HA as we
would like it.
I read also that filover time depends on the number of clients. We will
only have three, and they will not do heavy IO. So that should perhaps
help a bit.
Is there any difference between an 'uncontrolled' ceph server
(accidental) reboot, and a controlled reboot, where we (for example)
first failover the MDS in a controlled, gentle way?
MJ
Op 26-10-2022 om 14:40 schreef Eugen Block:
Just one comment on the standby-replay setting: it really depends on the
use-case, it can make things worse during failover. Just recently we had
a customer where disabling standby-replay made failovers even faster and
cleaner in a heavily used cluster. With standby-replay they had to
manually clean things up in the mounted directory. So I would recommend
to test both options.
Zitat von William Edwards <[email protected]>:
Op 26 okt. 2022 om 10:11 heeft mj <[email protected]> het volgende
geschreven:
Hi!
We have read https://docs.ceph.com/en/latest/man/8/mount.ceph, and
would like to see our expectations confirmed (or denied) here. :-)
Suppose we build a three-node cluster, three monitors, three MDSs,
etc, in order to export a cephfs to multiple client nodes.
On the (RHEL8) clients (web application servers) fstab, we will mount
the cephfs like:
cehp1,ceph2,ceph3:/ /mnt/ha-pool/ ceph
name=admin,secretfile=/etc/ceph/admin.secret,noatime 0 2
We expect that the RHEL clients will then be able to use (read/write)
a shared /mnt/ha-pool directory simultaneously.
Our question: how HA can we expect this setup to be? Looking for some
practical experience here.
Specific: Can we reboot any of the three involved ceph servers
without the clients noticing anything? Or will there be certain
timeouts involved, during which /mnt/ha-pool/ will appear
unresposive, and *after* a timeout the client switches monitor node,
and /mnt/ha-pool/ will respond again?
Monitor failovers don’t cause a noticeable disruption IIRC.
MDS failovers do. The MDS needs to replay. You can minimise the effect
with mds_standby_replay.
Of course we hope the answer is: in such a setup, cephfs clients
should not notice a reboot at all. :-)
All the best!
MJ
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