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

Eneko Lacunza elacunza at binovo.es
Fri May 29 09:58:29 CEST 2015


On 28/05/15 17:54, Dietmar Maurer wrote:
>> If you provide a buffer to the kernel, that you change while it is
>> working with it, I don't know why you expect a reliable/predictable
>> result? Specially (but not only) if you tell it not to make a copy!!
>>
>> Note that without O_DIRECT you won't get a "correct" result either; disk
>> may end not containing the data in the buffer when write was called.
>> Softmirror data will be identically uncertain :)
> Again, we do not expect a correct, reliable or predicable result. But we
> would expect that the RAID device is still in a consistent state. Or
> is that too much to expect?
>
I'm as surprised as you are :) but I understand if md/drbd layers say 
it's not their fault.

They could detect the O_DIRECT flag for consistent mirroring, but I 
don't know how easy/hard that would be.

It is also quite curious that DRBD layer has checks that detect this 
problem (but doesn't attempt to fix it).

Cheers
Eneko

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