Re: [RFC][PATCH 1/2] Add a super operation for writeback

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jun 03, 2014 at 04:05:31PM +0200, Jan Kara wrote:
> So we currently flush inodes in first dirtied first written back order when
> superblock is not specified in writeback work. That completely ignores the
> fact to which superblock inode belongs but I don't see per-sb fairness to
> actually make any sense when
> 1) flushing old data (to keep promise set in dirty_expire_centisecs)
> 2) flushing data to reduce number of dirty pages
> And these are really the only two cases where we don't do per-sb flushing.
> 
> Now when filesystems want to do something more clever (and I can see
> reasons for that e.g. when journalling metadata, even more so when
> journalling data) I agree we need to somehow implement the above two types
> of writeback using per-sb flushing. Type 1) is actually pretty easy - just
> tell each sb to writeback dirty data upto time T. Type 2) is more difficult
> because that is more openended task - it seems similar to what shrinkers do
> but that would require us to track per sb amount of dirty pages / inodes
> and I'm not sure we want to add even more page counting statistics...
> Especially since often bdi == fs. Thoughts?

Honestly I think doing per-bdi writeback has been a major mistake.  As
you said it only even matters when we have filesystems on multiple
partitions on a single device, and even then only in a simple setup,
as soon as we use LVM or btrfs this sort of sharing stops to happen
anyway.  I don't even see much of a benefit except that we prevent
two flushing daemons to congest a single device for that special case
of multiple filesystems on partitions of the same device, and that could
be solved in other ways.

The major benefit of the per-bdi writeback was that for the usual case
of one filesystem per device we get exactly one flusher thread per
filesystems intead of multiple competing ones, but per-sb writeback
would solve that just as fine.