Re: [Qemu-devel] sheepdog's "CoQueue overlapping_queue;"

[Hitoshi Mitake]

On Mon, Apr 4, 2016 at 10:04 PM, Paolo Bonzini <address@hidden> wrote:

I am curious about why overlapping_queue is required for sheepdog.
Overlapping requests have unspecified outcome so the CoQueue is not
necessary as long as the server doesn't crash or return an error.

Hitoshi, could you clarify this point for me?

The CoQueue is required because the sheepdog server can return an error in a case of overlapping requests. For handling copy on write and discard, the sheepdog server interprets requests that incur metadata (inode) update. It is required for garbage collecting unused objects efficiently. When reference of the metadata is updated (x -> y in a case of COW, x -> 0 in a case of discard), the server reclaims the old x object. If two or more requests try to update the reference of x, only first one request will be able to cause reclamation of x successfully. Second (or later) one's reclamation will fail because the x is already reclaimed by the first one.

This client side serialization is required because of the design of sheepdog. sheepdog doesn't have a dedicated metadata servers that can provide transactional update of metadata (e.g. HDFS has this kind of metadata server). So the clients need to serialize updates for avoiding inconsistent metadata update.

(Technically, sheepdog does transactional update of metadata in a case of virtual disk creation. It is based on virtual synchrony so the update can be transactional even without centralized metadata management component)

Thanks,

Hitoshi

Thanks,

Paolo