[rescue] Re: [SunHELP] Help with RAID

Curtis H. Wilbar Jr. rescue at hawkmountain.net
Thu Jun 5 11:42:05 CDT 2003

>Date: Thu, 5 Jun 2003 11:40:17 -0400
>From: Charles Shannon Hendrix <shannon at widomaker.com>
>To: "Curtis H. Wilbar Jr." <rescue at hawkmountain.net>, The Rescue List 
<rescue at sunhelp.org>
>Subject: Re: [rescue] Re: [SunHELP] Help with RAID
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>On Thu, Jun 05, 2003 at 12:18:07AM -0400, Curtis H. Wilbar Jr. wrote:
>> If you have the CPU... you'll get more space by going with RAID 5,
>> but your I/O throughput will go down.  Another instance of space vs speed.
>The killer is having enough bandwidth for the I/O overhead.

I haven't put much thought into it yet, but how is the I/O overhead
that much more than having the same number of individual drives on a bus ?

How does RAID 5 impact the bus ?  (Obviously one has to write the parity,
and without diving into SCSI protocol, I'm sure less information is required
to setup 10 Meg of transfer off one drive than 10 meg of transfer off a RAID
5 setup... but how significant is it ?

>If you have journaled raid5 (not seen it myself) its supposed to be 6
>ios per write.

I use DiskSuites trans device for journaling...

I have the oposite of optimum... I have 12 9 gig drives on ODS doing
software RAID 5 + journaling along with two mirrored 4 G drives for the
OS and a CDROM drive (all 15 targets !).

All powered by a Pentium 200MMX and 2940UW running Solaris for Intel.

Very stable, but the throughput is not high when writing to the RAID 5
(you have to calculate the parity).  I think the biggest reason it the
lack of CPU horsepower.

This is just a home fileserver, so space and data protection were more
concerns than speed, so RAID5 was chosen to maximize the space while
providing some protection for the data.

>However, I think some raid5 software and hardware optimizes things a
>bit, and hardware raid will be hiding the overhead from your host bus.
>If you have modern UW SCSI drives, you need a lot of bandwidth if you 
>want to maximize their raid5 throughput.
>If you have drives which can do 40MB/sec, then you can only put 8 of
>them on one 320MB/sec controller, perhaps it should be less.

Well, I'm way off on this mark.... 15 devies, 12 RAID5 on one Ultra2LVD
(80MB/S) bus...

>It seems like to me, if you have the controllers and the bandwidth,
>you will steadily gain speed by adding drives.  So far I've always had
>either hardware raid, or had enough CPU to make it noise in the signal,
>but it would be interesting to see how well some low-end CPUs with a lot
>of bandwidth could do with software raid5.
>Where I last worked, we had a huge number of drives in SMC and
>StorageTek raids, but the I/O controllers were 2G/sec each on SGI Origin
>systems, so the overhead was taken care of by brute force.
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-- Curt

Curtis Wilbar
Hawk Mountain Networks
rescue at hawkmountain.net

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