[sheepdog] Sheepdog kernel client

[Chris Webb]

A colleague and I have been discussing the possibility of using Sheepdog for
the storage backing physical hosts as well as qemu virtual machines. It
feels like it wouldn't be particularly hard to take the relatively simple
qemu <-> sheep protocol defined in qemu/block/sheepdog.c and write a kernel
block device, perhaps based on the existing linux nbd driver.

Whilst there aren't any obviously problems with mounting a block device
backed on sheepdog outside the cluster, I'm worried about mounts of sheepdog
block devices on hosts within the cluster, or even on a machine that's just
acting as a gateway. Am I right that this is unlikely to work?

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?

(But somehow, cluster filesystems like Gluster and Ceph have user mode
storage servers and are still able to mount the filesystem on the same nodes
as the storage. I'm puzzled that the same problem doesn't afflict them. Is
there some technique they use to avoid deadlock that would be applicable to
a Sheepdog kernel client?)

Cheers,

Chris.