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. 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. 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. 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. -- Regards, Kai Replies to list-only preferred. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
