Subject: Re: comparing raid-like filesystems
To: Antti Kantee , Jukka Marin <>
From: Jason R Thorpe <>
List: current-users
Date: 02/01/2003 08:05:23
On Sat, Feb 01, 2003 at 11:08:32AM +0200, Antti Kantee wrote:

 > > I don't know why, because when RAID is 100% busy, CPU load is almost
 > > zero and the disk load 25% or so.  It's like if RAIDframe had some
 > > usleep() calls in the code to make things go slower..
 > What's your stripe unit size? I also was suffering from inexplainable
 > slowness until I dropped the stripe unit size to 16 sectors. Before that
 > my RAID5 gave something like 5MB/s write speeds, now it's giving more
 > than 25MB/s.

Right, this is a deficiency in how we handle layered disk I/O.  Right
now we are limited to MAXPHYS (64k) for each "disk".  If this "disk"
is a RAID volume, then this 64k must be divided up among the underlying
components, so 64k / width.  This has two problems:

	* Since you can't write an entire stripe in one I/O, RAID5
	  has to perform extra I/O to update the parity.

	* You end up using small transfer to the underlying component

Reducing your stripe unit size will mitigate the former.  We need
infrastructure changes in the kernel to fix the latter.

        -- Jason R. Thorpe <>