Re: [PATCH V2] writeback: fix hung_task alarm when sync block

[Dave Chinner <david@fromorbit.com>]

On Wed, Jun 13, 2012 at 10:48:40PM +0800, Fengguang Wu wrote:
> > This really feels like we're papering over the problem.
> 
> That's true. The majority users probably don't want to cache 100s
> worth of data in memory. It may be worthwhile to add a new per-bdi
> limit whose unit is number-of-seconds (of dirty data).

Doesn't work. You have a BBWC that takes in 500MB of random 4k
writes in a second, then starts to flush and needs to do a RMW cycle
for every 4k write it cached. On RAID5/6, the flush rate will be
about 100 IOPS, so it could take half an hour to flush those writes
that took a second to dump into the cache. IO for that entire half
hour will be extremely slow, and if you isue a sync during it, then
that's when you get a hung task timer.

Limiting the amount of writeback to a few seconds of IO simply won't
fix this - the ingest rate of BBWCs is simply too great to prevent
such events by a slow moving bandwidth throttle....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com