Subject: Re: RAID In general (Re: Hot Swappable IDE Kits)
To: None <>
From: Chuck Yerkes <>
List: current-users
Date: 04/17/2003 14:52:13
Quoting Manuel Bouyer (
> On Wed, Apr 16, 2003 at 01:55:52PM -0600, Greg Oster wrote:
> > > 
> > > Yes, software raid is cheaper to acquire.  But it's slower
> > 
> > Do you have pointers to any specifics on this?  I'd be quite interested in 
> > seeing benchmarks for "modern" hardware... (preferably using the same disks 
> > on the same hardware)
> This is not true, at last for external boxes connected to a SCSI port.
> For example, in my DS20 I can get 50MB/s (though the filesystem, more from
> the raw device) out of a raid-1 built from 20 disks spread over multiple
> SCSI UW channels. An external box would be limited by the SCSI bandwidth 
> (something like 35MB/s in this case). Yes, the external raid box could

I have boxes that give me a REAL 45MB/s of RAID(3) writes.
(software) Stripe 3 of those on different PCI *busses* and getting
120MB or more of WRITES is doable (and done).

I've had several discussions of late with people who hate hardware
RAID not due to bad experience with RAID, but rather experiences with
*bad*RAID.  (3 folks have mentioned DEC/RAID boxes for Alphae).

The RAID I like has mirrored, battery backed SIMMS doing write
caching.  We've never filled that 256MB cache - it bleeds out faster
than they can come in under the most brutal conditions.
At ex-work, we tested and it performed the same as a SCSI SSD drive
(with far more storage :).

> be connected to a Ultra320 controller. I could as well use 20 U320 drives
> connected to 4 U320 controller, and it would still be faster.
> Also, in my setup I can have 20 * 256 = 5120 outstanding commands.
> With an external RAID this would be limited to 256.

I'm not sure I'd put 20 drives on a single PCI bus.  U320 is fine,
but PCI is smaller unless you have PCI/100 - only now becoming

> hardware RAID PCI controllers don't have the limitations of external raid
> boxes, as they have mutliple channels. So they should be at last as fast as
> software RAID.

But then you have a controller locked inside a box.  And configured.
Take a hit on that box, and you're screwed.  With FCAL, we even
get electrical isolation (I dislike FCAL for direct connect cause
its slow in practice and very costly - but Sun markets the hell
out of it, mostly based on the fact that it's so much faster than
Sun SCSI.  If you use non-Sun SCSI, that speed gap collapses :).

FCAL and SCSI both let me connect more than one host to it for
realtime or "close enough to realtime with intervention" recovery
should a server eat it (HA/Failover).