[pve-devel] Default cache mode for VM hard drives

[Eneko Lacunza]

Hi Stanislav,

I really think you should have a look where the problem is. This is not 
Proxmox specific at all, so this list doesn't seem the place to diagnose it.

If DRBD guys think it's an upper layer problem, maybe they can point you 
to the problematic layer, and then contact the mantainer (probably 
someone in kernel development).

I recall from ceph list that there were some barrier problems in kernels 
< 2.6.33 . I don't know whether those are fixed in the kernel from RHEL6 
Proxmox uses...

Cheers
Eneko

On 28/05/15 09:22, Stanislav German-Evtushenko wrote:
> Hi Dietmar,
>
> I did it couple of times already and everytime I had the same answer 
> "upper layer problem". Well, as we've done this long way up to this 
> point I would like to continue.
>
> I have just done the same test with mdadm and not DRBD. And what I 
> found that this problem was reproducible on the software raid too, 
> just as it was claimed by Lars Ellenberg. It means that problem is not 
> only related to DRBD but to O_DIRECT mode generally when we don't use 
> host cache and a block device reads data directly from userspace.
>
> The testcase is bellow.
>
> 1. Prepare
>
> dd if=/dev/zero of=/tmp/mdadm1 bs=1M count=100
> dd if=/dev/zero of=/tmp/mdadm2 bs=1M count=100
> losetup /dev/loop1 /tmp/mdadm1
> losetup /dev/loop2 /tmp/mdadm2
> mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/loop{1,2}
>
> 2. Write data with O_DIRECT
>
> ./a.out /dev/md0
>
> 3. Check consistency with vbindiff
>
> vbindiff /tmp/mdadm{1,2}      #press enter multiple times to skip metadata
>
> And here we find that data on "physical devices" is different and md 
> raid did not catch this.
>
>
> On Thu, May 28, 2015 at 7:40 AM, Dietmar Maurer <dietmar at proxmox.com 
> <mailto:dietmar at proxmox.com>> wrote:
>
>     > What this means?
>
>     I still think you should discuss that on the DRBD list.
>
>
> Best regards,
> Stanislav German-Evtushenko
>
>
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