On 2017-02-07 15:36, Kai Krakow wrote:
Am Tue, 7 Feb 2017 09:13:25 -0500
schrieb Peter Zaitsev <p...@percona.com>:
Hi Hugo,
For the use case I'm looking for I'm interested in having snapshot(s)
open at all time. Imagine for example snapshot being created every
hour and several of these snapshots kept at all time providing quick
recovery points to the state of 1,2,3 hours ago. In such case (as I
think you also describe) nodatacow does not provide any advantage.
Out of curiosity, I see one problem here:
If you're doing snapshots of the live database, each snapshot leaves
the database files like killing the database in-flight. Like shutting
the system down in the middle of writing data.
This is because I think there's no API for user space to subscribe to
events like a snapshot - unlike e.g. the VSS API (volume snapshot
service) in Windows. You should put the database into frozen state to
prepare it for a hotcopy before creating the snapshot, then ensure all
data is flushed before continuing.
Correct.
I think I've read that btrfs snapshots do not guarantee single point in
time snapshots - the snapshot may be smeared across a longer period of
time while the kernel is still writing data. So parts of your writes
may still end up in the snapshot after issuing the snapshot command,
instead of in the working copy as expected.
Also correct AFAICT, and this needs to be better documented (for most
people, the term snapshot implies atomicity of the operation).
How is this going to be addressed? Is there some snapshot aware API to
let user space subscribe to such events and do proper preparation? Is
this planned? LVM could be a user of such an API, too. I think this
could have nice enterprise-grade value for Linux.
Ideally, such an API should be in the VFS layer, not just BTRFS.
Reflinking exists in other filesystems already, it's only a matter of
time before they decide to do snapshotting too.
XFS has xfs_freeze and xfs_thaw for this, to prepare LVM snapshots. But
still, also this needs to be integrated with MySQL to properly work. I
once (years ago) researched on this but gave up on my plans when I
planned database backups for our web server infrastructure. We moved to
creating SQL dumps instead, although there're binlogs which can be used
to recover to a clean and stable transactional state after taking
snapshots. But I simply didn't want to fiddle around with properly
cleaning up binlogs which accumulate horribly much space usage over
time. The cleanup process requires to create a cold copy or dump of the
complete database from time to time, only then it's safe to remove all
binlogs up to that point in time.
Sadly, freezefs (the generic interface based off of xfs_freeze) only
works for block device snapshots. Filesystem level snapshots need the
application software to sync all it's data and then stop writing until
the snapshot is complete.
As of right now, the sanest way I can come up with for a database server
is to find a way to do a point-in-time SQL dump of the database (this
also has the advantage that it works as a backup, and decouples you from
the backing storage format).
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