Re: Help with low raid5 performance

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Mon, Jan 10, 2011 at 11:31:07AM -0600, Greg Oster wrote:
> 
> The problem is that with a 4-disk RAID 5 set you have (effectively) 3
> data disks.  As you can't get a nice power-of-two number to divide
> evenly by 3, so it doesn't matter too much what you pick -- performance
> is going to be greatly hindered by the small-write problem (as for a
> MAXPHYS write, the best you can do is one full-stripe write, and one
> partial-stripe write.)

FFS will limit itself to 48K writes if you tell it to.   Use a 16K FFS
blocksize and maxcontig=3.

Thor