Re: copying partition to partition, sector by sector, live

[Chris Mason <mason@suse.com>]

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