Re: Large write = large latency for small writes

[Peter Zijlstra]

On Tue, 2009-03-17 at 13:19 -0700, David Rees wrote:

> I have a simple test case which demonstrates the huge increase in
> write latency that occurs for small writes when a large disk
> saturating write is also in progress [3]:
> 
> dd if=/dev/zero of=/tmp/bigfile bs=1M count=10000 conv=fdatasync &
> sleep 10
> time dd if=/dev/zero of=/tmp/smallfile bs=4k count=1 conv=fdatasync
> 
> On a handful of systems I have access to, it took anywhere from 6-45
> seconds for the small write to complete.  Others in the bug have
> reproduced this across a number of filesystems (ext3, reiserfs, ext4).
> xfs in particular seems to handle this test case better than the
> others.  As do systems which can sustain high write speeds.

How does it fare without the fdatasync?

That is, is it the sync that's taking ages, or the ditry?

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