Hi,
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.
it really depends on what you're trying to achieve since there are
lots of different scenarios how to setup and configure one or more
ceph filesystems. And without testing your desired scenario you can't
really say that it's disappointing. ;-)
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.
In that case it's more likely that your clients won't notice a
failover, but again, test it.
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?
I haven't noticed a difference, but I'm still working with older
clusters (mostly Nautilus), maybe in newer versions the failover is
smoother, I can't tell yet.
Regards,
Eugen
Zitat von mj <lists@xxxxxxxxxxxxx>:
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 <wedwards@xxxxxxxxxxxxxx>:
Op 26 okt. 2022 om 10:11 heeft mj <lists@xxxxxxxxxxxxx> 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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