[Jfs-discussion] benchmark results

[tytso at mit.edu]

On Fri, Dec 25, 2009 at 08:22:38AM -0800, Larry McVoy wrote:
> 
> Dudes, sync() doesn't flush the fs cache, you have to unmount for that.
> Once upon a time Linux had an ioctl() to flush the fs buffers, I used
> it in lmbench.  
> 
> 	ioctl(fd, BLKFLSBUF, 0);
> 
> No idea if that is still supported, but sync() is a joke for benchmarking.

Depends on what you are trying to do (flush has multiple meanings, so
using can be ambiguous).  BLKFLSBUF will write out any dirty buffers,
*and* empty the buffer cache.  I use it when benchmarking e2fsck
optimization.  It doesn't do anything for the page cache.  If you are
measuring the time to write a file, using fsync() or sync() will
include the time to actually write the data to disk.  It won't empty
caches, though; if you are going to measure read as well as writes,
then you'll probably want to do something like "echo 3 >
/proc/sys/vm/drop-caches".

						- Ted