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

Eneko Lacunza elacunza at binovo.es
Thu May 28 14:43:34 CEST 2015

On 28/05/15 13:49, Dietmar Maurer wrote:
>> I'm not kernel/IO expert in any way, but I think this test program has a
>> race condition, so it is not helping us diagnose the problem.
>> We're writing to buffer x while it is in use by write syscall. This is
>> plainly wrong on userspace.
> For this test, we do not care about userspace semantics. The point is
> that it results in damaged RAID, which should never! happen.
I don't think the kernel can do anything if you're not complying with 
calling conventions.

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 :)


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