Theodore Tso wrote: > On Sat, Apr 19, 2008 at 02:07:34PM -0500, Eric Sandeen wrote: >> If you really just want to verify a snapshot of the fs at a point in >> time, surely there are simpler ways. If the device is on lvm, there's >> already a script floating around to do it in automated fasion. (I'd >> pondered the idea of introducing META_WRITE (to go with META_READ) and >> maybe lvm could do a "metadata-only" snapshot to be lighter weight?) > > That would be great, although I think the major issue is not > necessarily the performance problems of using an LVM snapshot on a > very busy filesystem well, backing space for the snapshot could be an issue too. Basically, if you're only using it for this purpose, why COW all the post-snapshot data if you just don't care... > (althouh I could imagine for some users this > might be an issue), but rather for filesystem devices that aren't > using LVM at all. (I've heard some complaints that LVM imposes a > performance penalty even if you aren't using a snapshot; has anyone > done any benchmarks of a filesystem with and without LVM to see > whether or not there really is a significant performance penalty; > whether or not there really is one, the perception is definitely out > there that it does.) I've heard from someone who did some testing about a minor penalty, but I can't point to any published test so I guess that's just more hearsay. It's intuitive that putting lvm on top of a block device might not be absolutely, 100% free, though.... Adds to stack, too. > If we could do a lightweight snapshot that didn't require an LVM, that > would be really great. But that's probably not an ext4 project, and > I'm not sure the it would be considered politically correct in the > LKML community. Yep; my original reply originally wished something about non-lvm snapshots but... while yes, it'd be nice for this purpose, ponies for everyone would be nice too... :) But I didn't mention it because... how do you do a generic non-lvm snapshot of, say, /dev/sda3 without some sort of volume manager...? If there's some clever idea that could be implemented cleanly, I'd be all ears. :) -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
