[sheepdog] Sheepdog kernel client

[Chris Webb]

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.

Maybe the problem runs too deep for this to work, though?

Cheers,

Chris.