Re: Does balance implicitly defrag?

[Paul Richards <paul.richards@gmail.com>]

On 8 July 2013 22:12, David Sterba <dsterba@suse.cz> wrote:
> On Tue, Jul 02, 2013 at 02:28:25PM +0100, Paul Richards wrote:
>> When I run "btrfs filesystem balance", does this implicitly
>> defragment the filesystem?  (Assuming there is plenty free space)
>
> In some sense it defragments the filesystem. The blockgroups are moved
> around based on the balance filter. Here a blockgroup consists of
> various and unrelated file extents. Extents representing a single file
> are not made contiguous, this is done by 'btrfs file defrag'.
>
> I think that the final layout of the 1G-chunks is not necessarily
> contiguous, ie. it depends on the chunk-allocator, the time of the
> allocation request and there's nothing like "preallocate 30G for whole
> balance and put all data together because the space is available".
>

Okay, I think I understand.  It sounds like balance will fix external
fragmentation (within the block groups, up to the 1G-chunk
granularity), and will not fix any internal fragmentation.

Thanks. :)




--
Paul Richards
@pauldoo