[Gluster-users] Strange file corruption

[Krutika Dhananjay]

----- Original Message -----

> From: "Lindsay Mathieson" <lindsay.mathieson at gmail.com>
> To: gluster-users at gluster.org
> Sent: Tuesday, December 8, 2015 7:29:00 AM
> Subject: Re: [Gluster-users] Strange file corruption

> Hi Udo, thanks for posting your volume info settings. Please note for the
> following, I am not one of the devs, just a user, so unfortunately I have no
> authoritative answers :(

> I am running a very similar setup - Proxmox 4.0, three nodes, but using ceph
> for our production storage. Am heavily testing gluster 3.7 on the side. We
> find the performance of ceph slow on these small setups and management of it
> a PITA.

> Some more questions

> - how are your VM images being accessed by Proxmox? gfapi? (Proxmox Gluster
> storage type) or by using the fuse mount?

> - whats your underlying filesystem (ext4, zfs etc)

> - Are you using the HA/Watchdog system in Proxmox?

> On 07/12/15 21:03, Udo Giacomozzi wrote:

> > esterday I had a strange situation where Gluster healing corrupted * all *
> > my
> > VM images.
> 

> > In detail:
> 
> > I had about 15 VMs running (in Proxmox 4.0) totaling about 600 GB of qcow2
> > images. Gluster is used as storage for those images in replicate 3 setup
> > (ie. 3 physical servers replicating all data).
> 
> > All VMs were running on machine #1 - the two other machines (#2 and #3)
> > were
> > * idle * .
> 
> > Gluster was fully operating (no healing) when I rebooted machine #2.
> 
> > For other reasons I had to reboot machines #2 and #3 a few times, but since
> > all VMs were running on machine #1 and nothing on the other machines was
> > accessing Gluster files, I was confident that this wouldn't disturb
> > Gluster.
> 
> > But anyway this means that I rebootet Gluster nodes during a healing
> > process.
> 

> > After a few minutes, Gluster files began showing corruption - up to the
> > point
> > that the qcow2 files became unreadable and all VMs stopped working.
> 
> :( sounds painful - my sympathies.

> You're running 3.5.2 - thats getting rather old. I use the gluster debian
> repos:

> 3.6.7 : http://download.gluster.org/pub/gluster/glusterfs/3.6/LATEST/Debian/
> 3.7.6 :
> http://download.gluster.org/pub/gluster/glusterfs/LATEST/Debian/jessie/

> 3.6.x is the latest stable, 3.7 is close to stable(?) 3.7 has some nice new
> features such as sharding, which is very useful for VM hosting - it enables
> much faster heal times.

> Regards what happened with your VM's, I'm not sure. Having two servers down
> should have disabled the entire store making it not readable or writable. I
> note that you are missing some settings that need to be set for VM stores -
> there will be corruption problems if you live migrate without them.

> quick-read=off
> read-ahead=off
> io-cache=off
> stat-prefetch=off
> eager-lock=enable
> remote-dio=enable
> quorum-type=auto
> server-quorum-type=server

Perfectly put. I am one of the devs who work on replicate module. You can alternatively enable this configuration in one shot using the following command for VM workloads: 
# gluster volume set <VOLNAME> group virt 

Like Lindsay mentioned, it is not clear why the brick on the first node did not go down when the other two nodes were rebooted, given that server quorum is enabled on your volume. 
Client-quorum option seems to be missing in your volume. Once you enable it, it is necessary for at least two bricks per replica set to be online for the writes to happen on the volume. 
If two nodes are down, the volume becomes read-only, causing the VMs to possibly go into a paused state. 

-Krutika 

> "stat-prefetch=off" is particularly important.

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