Re: [dm-devel] Snapshot target and DAX-capable devices
On Wed, Dec 12, 2018 at 05:43:22PM -0500, Mike Snitzer wrote:
> I would expect that dm-snapshot will be used quite a lot for
> short-lived snapshots (that only live during a database backup or an
> fsck run). I would hardly call that a "niche use case".
dm-snapshot is only ~60% performant for 1 snapshot. Try to do
additional snapshots and performance crawls to a stop (though I haven't
reassessed performance in a while).
dm-snapshot has been in Linux since before 2005, I don't know of all the
users of it -- maybe there are a ton of users who only take a single
temporary snapshot and we're all oblivious.
Well, here's one user:
https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/scrub/e2scrub.in
This is a spiffed up version of my original script:
https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/contrib/e2cronc...
I suppose we should look into enhancing e2scrub so it can deal with
volumes stored on dm-thin pools.....
dm-thinp has the concept of an "external origin". Changes
to origin LVM
volume get copied out to the thin-pool (same copy cost as old
dm-snapshot). But IIRC from that point on your LVM volume is a dm-thin
device.
I would think the *snapshot* would have to be the dm-thin device, not
the origin volume, correct?
The original LVM module would still be mounted through the original
LVM device-mapper device, so it couldn't get transmogrified to be a
dm-thin device, right?
- Ted