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

[Stanislav German-Evtushenko]

On Thu, Nov 20, 2014 at 10:49 AM, Cesar Peschiera <brain at click.com.py>
wrote:

> Cache=none means no host cache but backend cache is still in use. In case
>> of DRBD this is a buffer in DRBD. So O_DIRECT return OK when data
>> reaches this buffer and not RAID cache.
>>
>
> Excuse me please if i intervene in this conversation, but as i understand,
> if the data is in a buffer of DRBD, then DRBD must know that there exist
> data to replicate, so obviuosly the problem isn't in the upper layers (KVM,
> any buffer in the RAM controlled by some software, etc.), so the buffer of
> DRBD should be optimized according to convenience.
>
> Moreover, DRBD have several Web pages that tell us with great detail about
> of optimize many things, including the configuration of his buffers for
> avoid the data loss, also with examples of with and without a RAID
> controller in the middle. So it, the softwares that are in the upper layers
> nothing can do about since that DRBD takes the control of the data as also
> of his own buffer.
>
>
If we enable integrity checking (DRBD will compare checksums for all blocks
prior committing to backand) in DRBD while using cache=none for DRBD then
we cat this kind of messages from time to time:
block drbd0: Digest mismatch, buffer modified by upper layers during write:
25715616s +4096

Stanislav
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