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

[Josef Bacik <jbacik@fb.com>]

On 09/14/2014 12:46 AM, Wang Shilong wrote:
> Hi Josef,
>
>> 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,
>
> I am wondering whether we could check if it a Mixed Mode, we don’t need remove
> Mixed mode block groups automatically…

Good point, I'll update the patch.  Thanks,

Josef