[rescue] Re: [SunHELP] Help with RAID

[Charles Shannon Hendrix]

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.

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

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.

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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