Re: Page Cache writeback too slow, SSD/noop scheduler/ext2

[Wu Fengguang <fengguang.wu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Tue, Mar 31, 2009 at 08:16:52PM +0800, Jos Houtman wrote:
> > 
> > Next to that I was wondering if there are any plans to make sure that not
> > all dirty-files are written back in the same interval.
> > 
> > In my case all database files are written back each 30 seconds, while I
> > would prefer them to be more divided over the interval.
> 
> There another question I have: does the writeback go through the io
> scheduler? Because no matter the io scheduler or the tuning done, the
> writeback algorithm totally starves the reads.

I noticed this annoying writes-starve-reads problem too. I'll look into it.

> See the url below for an example with CFQ, but deadline or noop all show
> this behaviour:
> http://94.100.113.33/535450001-535500000/535451701-535451800/535451800_6_L7g
> t.jpeg
> 
> Is there anything I can do about this behaviour by creating a better
> interleaving of the reads and writes?

I guess it should be handled in the generic block io layer.  Once we
solved the writes-starve-reads problem, the bursty-writeback behavior
becomes a no-problem for SSD.

Thanks,
Fengguang

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