Re: btrfs defra

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.

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