Re: clearing filesystem cache for I/O benchmarks

[Hans Reiser <reiser@namesys.com>]

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