On Mon, Jun 13, 2011 at 04:50:37PM +0200, Bernhard Duebi wrote: > I'm no developer, just a simple user. I use btrfs as the root filesystem > on my laptop. I use VirtualBox. Every VB disk lives in a separat btrfs > subvolume. I use btrfs snapshots for backup and to clone VB VMs. > > I wonder what happens when I defrag a file that is part of multiple > snapshots? Will all instances of this file be defragmented, or will the > file get its own fresh copy?
The file gets its own copy. > If it is important, my system is a 64bit ubuntu 10.10 and the filesystem > is mounted with the option compress. If it helps, I would consider to > upgrade to 11.04. > > BTW: what is the difference between defrag and balance ? Defrag works on individual files, and tries to find a contiguous sequence of bytes to write the file's data to. In the process, the current implementation will break any CoW duplication -- either within single files (copies with cp --reflink=always) or files copied via snapshotting. Balance works at a much higher level, rewriting all (or large quantities) of the filesystem data and metadata. Balance was originally designed to re-allocate all of the space the FS needs, thus enabling you to add more disks to a filesystem and ensure that they get used. (It's also useful for re-building spare copies of data if a disk's been removed as a result of failure). Balance will maintain all existing file semantics as it runs -- CoW copies are preserved, and no additional space will be used. Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- Someone's been throwing dead sheep down my Fun Well ---
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