Re: howto combat highly pathologic latencies on a server?

[Christoph Hellwig <hch@infradead.org>]

On Wed, Mar 10, 2010 at 06:17:42PM +0100, Hans-Peter Jansen wrote:
> While this system usually operates fine, it suffers from delays, that are 
> displayed in latencytop as: "Writing page to disk:     8425,5 ms": 
> ftp://urpla.net/lat-8.4sec.png, but we see them also in the 1.7-4.8 sec 
> range: ftp://urpla.net/lat-1.7sec.png, ftp://urpla.net/lat-2.9sec.png, 
> ftp://urpla.net/lat-4.6sec.png and ftp://urpla.net/lat-4.8sec.png.
> 
> >From other observations, this issue "feels" like it is induced by single 
> syncronisation points in the block layer, eg. if I create heavy IO load on 
> one RAID array, say resizing a VMware disk image, it can take up to a 
> minute to log in by ssh, although the ssh login does not touch this area at 
> all (different RAID arrays). Note, that the latencytop snapshots above are 
> made during normal operation, not this kind of load..

I had very similar issues on various systems (mostly using xfs, but some
with ext3, too) using kernels before ~ 2.6.30 when using the cfq I/O
scheduler.  Switching to noop fixed that for me, or upgrading to a
recent kernel where cfq behaves better again.