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Subjectfsync on large files
Hi kernel freaks.

I would appreciate a suggestion how to aproach this problem:

A customer is reporting performance problems which might be
related to fsync() on big files ...

Background:
This is a database (consisting of a few big files) which each
have a size 300-800 MB. They are synced regulary (currently each
minute) using fsync() to ensure that old transaction log entries
can be discarded safely.

Problem:
The fsync() seems to takes rather long and other kernel activity
seems to stall while the fsync() is processed (~3-5 seconds).
If the fsync() is disabled, this effect is said to disappear.
This is 2.0.3[56] on an UP box.

I remember it was mentioned in the past that ext2 fsync() has a
problem with fsync on large files (there was a patch from Scott Laird
<laird@pacificrim.net> which reverted to file_fsync() depending on
file size).

Is this still valid (i would guess so since the 2.2.x code is
basically the same), would it help to change to SMP (i'd guess not
- first thing that 2.2.x does ist lock_kernel()) or is avoiding the
ext2 fsync mechanism (as Scotts patch does) the best option?

Other options i can imagine: Use more (and smaller files) - the
effects is said to become noticeable if file size extends 200 MB.
Or is this effect unrelated and i should look elsewhere?


Any suggestions?
Michael

--
Michael Marxmeier Marxmeier Software GmbH
E-Mail: mike@msede.com Besenbruchstrasse 9
Voice : +49 202 2431440 42285 Wuppertal, Germany
Fax : +49 202 2431420 http://www.msede.com/

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