[sheepdog] Sheepdog kernel client

[MORITA Kazutaka]

At Mon, 1 Oct 2012 13:53:55 +0100,
Chris Webb wrote:
> 
> MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp> writes:
> 
> > Chris Webb wrote:
> > > 
> > > I remember that loopback iscsi and nbd are very prone to deadlock under
> > > memory pressure, because more dirty pages need to be created to be able to
> > > progress with writing out the existing ones. Presumably a kernel sheepdog
> > > driver would suffer from the same problem, and it would be very hard to
> > > enable sheepdog hosts to mount filesystems on a cluster of which they're a
> > > part?
> > 
> > Probably, the answer is yes...  I thought that it would be nice to
> > access a sheepdog gateway on localhost with an iSCSI protocol, but it
> > would lead to the deadlock problem.
> [...] 
> > Seems that Ceph suffers from the same problem:
> >   http://tracker.newdream.net/issues/3076
> 
> I wonder whether memory cgroups will eventually provide a mechanism that can
> help here. Perhaps a restricted container on the host could safely access
> the sheepdog-backed block device because it is constrained by memcg to never
> be able to dirty enough pages that the host is unable to make progress.
> Private filesystem namespaces could be used to explicitly ensure the mount
> isn't accidentally touched outside the memcg restriction.

So sheeps can allocate memory without flushing pagecache of sheepdog
VDIs?  Sounds like a reasonable way to cope with this problem.

Thanks,

Kazutaka