[libvirt] [RFC v2] external (pull) backup API

[John Snow]

On 04/12/2018 08:26 AM, Vladimir Sementsov-Ogievskiy wrote:
> 1. It looks unsafe to use nbd server + backup(sync=none) on same node,
> synchronization is needed, like in block/replication, which uses
> backup_wait_for_overlapping_requests, backup_cow_request_begin,
> backup_cow_request_end. We have a filter driver for this thing, not yet
> in upstream.

Is it the case that blockdev-backup sync=none can race with read
requests on the NBD server?

i.e. we can get temporarily inconsistent data before the COW completes?
Can you elaborate?

> 2. If we use filter driver anyway, it may be better to not use backup at
> all, and do all needed things in a filter driver.

if blockdev-backup sync=none isn't sufficient to get the semantics we
want, it may indeed be more appropriate to just leave the entire task to
a new filter node.

> 3. It may be interesting to implement something like READ_ONCE for NBD,
> which means, that we will never read these clusters again. And after
> such command, we don't need to copy corresponding clusters to temporary
> image, if guests decides to write them (as we know, that client already
> read them and don't going to read again).

That would be a very interesting optimization indeed; but I don't think
we have any kind of infrastructure for such things currently. It's
almost like a TRIM on which regions need to perform COW for the
BlockSnapshot.