Re: [libvirt] [RFC v2] external (pull) backup API
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.