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SubjectRe: Unices are created equal, but ...
Date
> 
> Whole Linux seems to be a memory file system ;-) They are caching
> like hell. Only benchmarks like bonnie on files of about 3xRAMSIZE
> address the fact that we want to bench the disk and not the RAM.
> Gerard compares chicken with eggs. This Byte Bench is really
> questionable.
>
The most evil things about aggressively write cache are the memory starvation
and sync issues. On FreeBSD we purposely decided to limit the amount of
dirty cached file data. It can become a real problem with big memory
systems!!! The AT&T GIS/NCR/Tandem boxes really acted badly when users
would tune the system to allow too much memory to be dirty filesystem
cache buffers. I could be convinced that 1/4 of memory for dirty buffers
is okay, and in some cases even more could be considered. But those cases
where a system could gain significantly from huge write caches are few
and far between. I guess if managed VERY WELL, a large (>1/2 mem) write cache
would be good -- I just haven't seen that yet.

John
dyson@freebsd.org


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