[sheepdog-users] cache flush: all or nothing

[Joseph Glanville]

On Thu, Jun 6, 2013 at 2:18 PM, MORITA Kazutaka
<morita.kazutaka at gmail.com> wrote:
> At Thu, 06 Jun 2013 23:04:01 +0800,
> Liu Yuan wrote:
>>
>> >
>> > Remember that I saw almost no network activity on the node after I've
>> > been writing data to the guest (max 100kbit/s).
>>
>> So FLUSH wasn't issued from VM
>
> I suspect that Valerio uses ext3 and an older kernel (earlier than
> Linux 3.1).  Then file system barriers are not enabled by default and
> fsync() inside the guest OS doesn't issue flushes at all.  Or,
> Perhaps, Valerio might set 'barrier=0' manually to the mount option.
>
> We must not assume that the guest OSes issues flush requests
> periodically.  It completely depends on how the users configure their
> file systems.

Actually doing anything else will violate the behavior the guest expects.
Sheepdog is meant to act like a block device, it's not sheepdogs fault
if the filesystem doesn't issue a FUA/FLUSH request.

>
> Thanks,
>
> Kazutaka
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