Messages in this thread | | | Date | Wed, 9 Jul 2008 22:27:01 -0700 | From | Andrew Morton <> | Subject | Re: Higher than expected disk write(2) latency |
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On Sat, 28 Jun 2008 14:11:32 +0200 Martin Lucina <mato@kotelna.sk> wrote:
> we're getting some rather high figures for write(2) latency when testing > synchronous writing to disk. The test I'm running writes 2000 blocks of > contiguous data to a raw device, using O_DIRECT and various block sizes > down to a minimum of 512 bytes. > > The disk is a Seagate ST380817AS SATA connected to an Intel ICH7 > using ata_piix. Write caching has been explicitly disabled on the > drive, and there is no other activity that should affect the test > results (all system filesystems are on a separate drive). The system is > running Debian etch, with a 2.6.24 kernel. > > Observed results: > > size=1024, N=2000, took=4.450788 s, thput=3 mb/s seekc=1 > write: avg=8.388851 max=24.998846 min=8.335624 ms > 8 ms: 1992 cases > 9 ms: 2 cases > 10 ms: 1 cases > 14 ms: 1 cases > 16 ms: 3 cases > 24 ms: 1 cases
stoopid question 1: are you writing to a regular file, or to /dev/sda? If the former then metadata fetches will introduce glitches.
stoopid question 2: does the same effect happen with reads?
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