On Mon, 2002-04-22 at 19:24, Phil Howard wrote: > On Mon, Apr 22, 2002 at 11:47:55PM +0100, Matthew Toseland wrote: > > | On Sun, Apr 21, 2002 at 08:39:55PM -0500, Phil Howard wrote: > | > On Mon, Apr 22, 2002 at 02:07:33AM +0100, Matthew Toseland wrote: > | > > | > | Ummm, LVM snapshots? (man lvcreate). > | > > | > No. Nothing to do with LVM. > | I was suggesting a solution. Your problem is that reiserfs's metadata is > | so dynamic that if you copy the partition while it is active, you end up > | with metadata loss, which has to be fixed by reiserfsck. A possible > | solution is to get a consistent snapshot. To do this, do: > | > | lvcreate -n <snapshotname> -L 500M -v /dev/<vgname>/<partition name> > | dd if=/dev/<vgname>/snapshotname of=/usr/local/temp/snapshot.img bs=1M > | lvremove -f /dev/<vgname>/snapshotname > | > | (1) 500M could be anything; it is the amount of space needed to log all > | changes to the partition while the dd is going on; you can extend it later > | but once it fills up completely, you're scr00d; presumably the dd will > | return an error > > Sounds like journaling at the sector level. > > How does all that change get replayed after the snapshot is done?
The snapshots use a simple copy on write setup. Before changing a block on the source, the original is copied to the snapshot. This allows for very fast snapshot creation, and a moderate runtime cost doing the copies. When you're done with the backup, you can just delete the snapshot without affecting the (now modified) original. > > I'm starting to think it might be better to go back to using rsync on > mounted filesystems. It might. Snapshots are very useful, but are best in databases and other setups where you need to freeze the FS at a specific point in time, and when you need an absolute minimum of application down time. -chris
