[sheepdog-users] cache flush: all or nothing

[MORITA Kazutaka]

At Thu, 6 Jun 2013 15:16:59 -0700,
Joseph Glanville wrote:
> 
> 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.

I think the background pusher, which is mentioned by Yuan, doesn't
harm the guest.  The feature is necessary sooner or later to reduce
the impact of vdi flush.

Thanks,

Kazutaka