Very bad FreeBSD SCSI RAID5 write speed performance

[Bruce Evans]

On Thu, 13 Nov 2003, Allen Briggs wrote:

> On Thu, Nov 13, 2003 at 06:38:53AM -0500, Gary Stanley wrote:
> > root at 64:[/tmp/blah]>dd if=/dev/zero of=/tmp/blah/blah
> > 89472+0 records in
> > 89471+0 records out
> > 45809152 bytes transferred in 8.546312 secs (5360108 bytes/sec)
>
> I suspect you know this, but if you give a larger block size (bs=...),
> you should see better performance.  Writing to a file on the filesystem
> not only brings up the softupdates question, but also your choices for
> block and fragment sizes.  On a RAID, there's also the question of
> stripe size and how that's set with respect to the filesystem block
> and fragment sizes.

Writing through the file system will cause reblocking so that the small
blocks are only seen by the software, especially for big files as in the
above, and since CPUs are now much faster than disks there should be
very little decrease in i/o speed due to using small blocks.  Some CPU
cycles will just become unavailable for other things.

Bruce