Re: [PATCH][v2] btrfs: add an ioctl to force chunk allocation

["Holger Hoffstätte" <holger@applied-asynchrony.com>]

On 8/5/19 5:41 PM, David Sterba wrote:
> On Mon, Aug 05, 2019 at 04:10:39PM +0200, Holger Hoffstätte wrote:
>> On 8/5/19 3:19 PM, Josef Bacik wrote:
>>> In testing block group removal it's sometimes handy to be able to create
>>> block groups on demand.  Add an ioctl to allow us to force allocation
>>> from userspace.
>>>
>>> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>>> ---
>>> v1->v2:
>>> - I noticed last week when backporting this that btrfs_chunk_alloc doesn't
>>>     figure out the rest of the flags needed for the type.  Use
>>>     btrfs_force_chunk_alloc instead so that we get the raid settings for the alloc
>>>     type we're using.
>>
>> Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
>>
>> Now works as intended - very nice, thanks!
> 
> Tell me, are you interested in this ioctl because it was the missing bit
> for testing or because the chunk allocator is so bad that you need a
> command to make work?

Mostly curiosity. "To make [it] work" would be a bit strong - btrfs works
fine by now. But yes, slightly over-allocating both metadata and data
(both by ~1-2 chunks) helps esp. when there is a mix of small and large
files, and to reduce fragmentation of large files. That should not be
surprising.

-h