Re: clearing filesystem cache for I/O benchmarks

[Tim Wright <timw@splhi.com>]

Take a look at the code in hdparm tool that handles the '-f' option.

Basically calling ioctl(fd, BLKFLSBUF, o) where fd is a file descriptor
opened on the block device on which your filesystem resides should be
enough to clear the cache.

Regards,

Tim

On Fri, 2004-07-23 at 15:54, Benjamin Rutt 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?
> 
> Unfortunately, I cannot:
> 
> 1) reboot the system
> 
> 2) re-mount the filesystem where the reads are occuring
> 
> So I propose that I am left with the following options:
> 
> 3) Reading through a file sufficiently larger than the RAM installed
>    on the system?  e.g. read through a 10GB file on a machine with 8GB
>    of RAM
> 
> 4) Since I can create the files fresh every time, I would write() them
>    out using O_DIRECT flag to open(), then the immediately following
>    read of that file would be guaranteed to avoid pulling it from
>    cache.
> 
> So, can someone evaluate whether how whether options 3 and 4 would
> work, or offer other suggestons?  And I wouldn't object if the issue
> of clearing disk and controller cache entered into the discussion (I'm
> thinking #3 would do a better job at clearing disk/controller caches).
> 
> In case it is relevant, here are the two relevant kernel versions I'm
> using, both under the distribution "Red Hat Enterprise Linux AS
> release 3 (Taroon)":
> 
>     Linux xio11 2.6.6 #2 SMP Wed Jun 9 10:37:24 EDT 2004 i686 i686 i386 GNU/Linux
>     
>     Linux xio06 2.4.21-9.ELhugemem #1 SMP Tue Apr 27 13:52:32 EDT 2004 i686 i686 i386 GNU/Linux
>     
> Thank you,
-- 
Tim Wright <timw@splhi.com>
Splhi