Re: clearing filesystem cache for I/O benchmarks

[Benjamin Rutt <rutt.4+news@osu.edu>]

Andrew Morton <akpm@osdl.org> writes:

> Benjamin Rutt <rutt.4+news@osu.edu> wrote:
>>
>>  How can I purge all of the kernel's filesystem caches, so I can trust
>>  that my I/O (read) requests I'm trying to benchmark bypass the kernel
>>  filesystem cache?
>
> Either delete the benchmark test files or

I'm not sure I follow.  If I delete the benchmark files, I'll only
need to create them again later in order to do a read test, and I'll
have the same problem then, of how to eliminate the just-written-data
from cache.  Unless you're suggesting I write using some special mode
that won't enter the written data into cache?  (e.g. O_DIRECT?)

> , in 2.6, use fsync+posix_fadvise(POSIX_FADV_DONTNEED);

Thanks for the reference, I wasn't aware of that one.  We are running
some 2.4 kernels in our storage cluster unfortunately so that won't be
usable for us everywhere.  I take it POSIX_FADV_DONTNEED is ignored
under 2.4.

A related question...if no posix_fadvise() advice has been given, does
reading sequentially every byte of an 8GB file on a machine with <=
8GB of RAM guarantee that any page cache data that existed on the
machine prior to the start of the 8GB read is now gone?
-- 
Benjamin Rutt