Re: [PATCH] writeback: plug writeback at a high level

[Chris Mason <chris.mason@fusionio.com>]

Quoting Dave Chinner (2013-06-14 22:50:50)
> From: Dave Chinner <dchinner@redhat.com>
> 
> Doing writeback on lots of little files causes terrible IOPS storms
> because of the per-mapping writeback plugging we do. This
> essentially causes imeediate dispatch of IO for each mapping,
> regardless of the context in which writeback is occurring.
> 
> IOWs, running a concurrent write-lots-of-small 4k files using fsmark
> on XFS results in a huge number of IOPS being issued for data
> writes.  Metadata writes are sorted and plugged at a high level by
> XFS, so aggregate nicely into large IOs. However, data writeback IOs
> are dispatched in individual 4k IOs, even when the blocks of two
> consecutively written files are adjacent.
> 
> Test VM: 8p, 8GB RAM, 4xSSD in RAID0, 100TB sparse XFS filesystem,
> metadata CRCs enabled.
> 
> Kernel: 3.10-rc5 + xfsdev + my 3.11 xfs queue (~70 patches)

I'm a little worried about this one, just because of the impact on ssds
from plugging in the aio code:

https://lkml.org/lkml/2011/12/13/326

How exactly was your FS created?  I'll try it here.

-chris