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SubjectRe: clearing filesystem cache for I/O benchmarks
Benjamin Rutt wrote:

>Hans Reiser <reiser@namesys.com> writes:
>
>
>
>>when benchmarking, please be careful that you don't end up
>>benchmarking umount/mount, or sync, or..... it can be remarkably hard
>>to avoid such mistakes.....
>>
>>
>
>I agree, I've made some blunders like that in the past. However for
>write tests, we are including fsync() time, once, at the end of a file
>write, since I feel it's unfair to trim that time.
>
fsync performance gives you different performance. Better to write more
stuff to flush the cache.

> Not including
>fsync() time would only test the ability of the various parts of the
>I/O systems to do write buffering. It's easy to do lots of write
>buffering, if you buy enough memory. Forcing the disks to write is
>the only fair way to compare writes between I/O systems.
>
>
It isn't fair. fsync is a different code path, and may be less
efficient. Or more, depending on the fs. reiser4 is currently not well
optimized for fsync, maybe next year I will change that but not this
week....
Benchmarking well is hard.....
Hans
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