Re: [PATCH] Btrfs: remove empty block groups automatically

[Chris Mason <clm@fb.com>]

On 09/12/2014 03:18 PM, Josef Bacik wrote:
> One problem that has plagued us is that a user will use up all of his space with
> data, remove a bunch of that data, and then try to create a bunch of small files
> and run out of space.  This happens because all the chunks were allocated for
> data since the metadata requirements were so low.  But now there's a bunch of
> empty data block groups and not enough metadata space to do anything.  This
> patch solves this problem by automatically deleting empty block groups.  If we
> notice the used count go down to 0 when deleting or on mount notice that a block
> group has a used count of 0 then we will queue it to be deleted.
> 
> When the cleaner thread runs we will double check to make sure the block group
> is still empty and then we will delete it.  This patch has the side effect of no
> longer having a bunch of BUG_ON()'s in the chunk delete code, which will be
> helpful for both this and relocate.  Thanks,

Thanks Josef, we've needed this forever.  I'm planning on pulling it in
for integration as well.

-chris