On Sat, 2006-05-13 at 13:00 -0400, Jiann-Ming Su wrote: > On 5/13/06, Arafangion <[EMAIL PROTECTED]> wrote: > > > > What happens if the data changes during your sync? This is why people > > are looking at using LVM snapshots - during which rsync'ing would be a > > good idea > > > > How does the LVM snapshot get around the problem of a changing > filesystem during the snapshot?
I'm not sure if you're asking about snapshots in general, or just about files changes during the moment when the snapshot is created. I'm not sure about the latter, but creating an LVM snapshots takes only a few seconds so I'd imagine it's not much of an issue. If you're asking about snapshots in general, however, that's the whole point of having snapshots. LVM takes a chunk of available space from the drive and designates it a snapshot diff area. Once the snapshot is activated, the current filesystem is "frozen" in place. No further changes are written to the original filesystem. Any future changes are written directly to the snapshot diff area. LVM transparently keeps track of both of these areas and returns a "unified" filesystem to the OS that includes all of the changes that have been made since the snapshot. However, if you ever decide to go back to the time of the snapshot, LVM just dumps the diff area and starts using the original image again. If, later, you decide that you want to keep all of the changes, LVM merges those changes in with the original filesystem and you're left with an up-to-date filesystem. It's kind of like having a CVS for filesystems. It's one of the greatest strengths of LVM in my opinion. (Second only to pairing it with xfs and allowing growing the filesystem without unmounting.) -- Alex Malinovich Support Free Software, delete your Windows partition TODAY! Encrypted mail preferred. You can get my public key from any of the pgp.net keyservers. Key ID: A6D24837
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