On Sun, 6 Mar 2022, Strahil Nikolov wrote:
It seems that only vh1-4 provide bricks, so vh5,6,7,8 can be removed.
Right, that was the point of my question: how to properly shutdown any one
of vh1-4 for maintenance without disrupting any VMs that may be running on
any of vh1-8.
When I had done a test of taking vh1 offline several days ago, all of the
VMs on vh4 went root-fs-read-only, which suprised me. I suppose it's
possible that there was something else at play that I haven't realized,
and that taking the vh1 gluster peer offline was not the root cause of the
vh4 VM failure. I haven't yet tried another such test yet - I was holding
off until I'd gotten some advice here first.
First check why vh5 is offline. Changes to all modes are propagated and in
this case vh5 is down and won't receive the peer detach commands.
Ok, interesting, but I have to admit that I don't understand that
requirement. I knew that vh5 was offline but I didn't know that I'd have
to bring it back online in order to properly shutdown one of vh1-4. Are
you certain about that? That is, if vh5 stays offline and I take vh4
offline, and then I bring vh5 online, will the quorum of peers not set vh5
straight?
Once you fix vh5, you can safely 'gluster peer detach' any of the nodes that
is not in the volume.
Ok, I'll try peer detach to take any of vh1-4 offline in a controlled
manner.
I take this to mean that if any one of the vh1-4 replica members were to
go offline in a uncontrolled manner, gluster peers may have a problem
which could lead to the sort of VM behaviour that I experienced. Frankly
this suprises me - I expected that my setup was more resilient in the face
of losing gluster replica members as long as there was still a quorum of
members operating normally.
Keep in mind that it's always best practice to have odd number of nodes in
the TSP (3,5,7,9,etc).
Do you know why that's the case? I understand that 3 or more are
recommended (could be 2 and a arbiter) but why an odd number? What
benefit does 3 provide that 4 does not?
Thanks,
Todd
Best Regards,
Strahil Nikolov
On Sun, Mar 6, 2022 at 4:06, Todd Pfaff
<pfaff@xxxxxxxxxxxxxxxxx> wrote:
[root@vh1 ~]# gluster volume info vol1
Volume Name: vol1
Type: Replicate
Volume ID: dfd681bb-5b68-4831-9863-e13f9f027620
Status: Started
Snapshot Count: 0
Number of Bricks: 1 x 4 = 4
Transport-type: tcp
Bricks:
Brick1: vh1:/pool/gluster/brick1/data
Brick2: vh2:/pool/gluster/brick1/data
Brick3: vh3:/pool/gluster/brick1/data
Brick4: vh4:/pool/gluster/brick1/data
Options Reconfigured:
transport.address-family: inet
nfs.disable: on
performance.client-io-threads: off
[root@vh1 ~]# gluster pool list
UUID Hostname State
75fc4258-fabd-47c9-8198-bbe6e6a906fb vh2 Connected
00697e28-96c0-4534-a314-e878070b653d vh3 Connected
2a9b891b-35d0-496c-bb06-f5dab4feb6bf vh4 Connected
8ba6fb80-3b13-4379-94cf-22662cbb48a2 vh5 Disconnected
1298d334-3500-4b40-a8bd-cc781f7349d0 vh6 Connected
79a533ac-3d89-44b9-b0ce-823cfec8cf75 vh7 Connected
4141cd74-9c13-404c-a02c-f553fa19bc22 vh8 Connected
On Sat, 5 Mar 2022, Strahil Nikolov wrote:
> Hey Todd,
>
> can you provide 'gluster volume info <VOLUME>' ?
>
> Best Regards,
> Strahil Nikolov
>
> On Sat, Mar 5, 2022 at 18:17, Todd Pfaff
> <pfaff@xxxxxxxxxxxxxxxxx> wrote:
> I have a replica volume created as:
>
> gluster volume create vol1 replica 4 \
> host{1,2,3,4}:/mnt/gluster/brick1/data \
> force
>
>
> All hosts host{1,2,3,4} mount this volume as:
>
> localhost:/vol1 /mnt/gluster/vol1 glusterfs defaults
>
>
> Some other hosts are trusted peers but do not contribute bricks, and
> they
> also mount vol1 in the same way:
>
> localhost:/vol1 /mnt/gluster/vol1 glusterfs defaults
>
>
> All hosts run CentOS 7.9, and all are running glusterfs 9.4 or 9.5
from
> centos-release-gluster9-1.0-1.el7.noarch.
>
>
> All hosts run kvm guests that use qcow2 files for root filesystems
that
> are stored on gluster volume vol1.
>
>
> This is all working well, as long as none of host{1,2,3,4} go
offline.
>
>
> I want to take one of host{1,2,3,4} offline temporarily for
> maintenance.
> I'll refer to this as hostX.
>
> I understand that hostX will need to be healed when it comes back
> online.
>
> I would, of course, migrate guests from hostX to another host, in
which
> case hostX would then only be participating as a gluster replica
brick
> provider and serving gluster client requests.
>
> What I've experienced is that if I take one of host{1,2,3,4} offline,
> this
> can disrupt some of the VM guests on various other hosts such that
> their
> root filesystems go to read-only.
>
> What I'm looking for here are suggestions as to how to properly take
> one
> of host{1,2,3,4} offline to avoid such disruption or how to tune the
> libvirt kvm hosts and guests to be sufficiently resilient in the face
> of
> taking one gluster replica node offline.
>
> Thanks,
> Todd
> ________
>
>
>
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