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

[Stanislav German-Evtushenko]

Hi Eneko,

Writing in QEMU-KVM are not thread-safe. I don't know if this is "by
design" or just a bug but proving this information here is necessary to
show that we should find a solution or work around for Proxmox.

General problem is that using Proxmox VE with default settings can make any
of SoftRAID-like inconsistent and we even would not know it. So far I know
that the issue happens on DRBD and MD RAID but possibly it also can happen
to many others, like Ceph, etc. In case of MD RAID we don't even have an
official tool to ensure data integrity.

The general problem described above can be addressed by adding an option
"cache=" for the storage object in Proxmox VE where you can set default
cache mode for all images on that storage. And it would be even better if
we could restrict a list of possible cache modes for a particular storage
object.

Best regards,
Stanislav


> 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
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