RAID-3?

[Greg 'groggy' Lehey]

On Wednesday, 18 August 2004 at 23:28:48 -0700, John-Mark Gurney wrote:
> Greg 'groggy' Lehey wrote this message on Thu, Aug 19, 2004 at 15:52 +0930:
>>> Your quoted text also seems a bit subjective as there are very valid
>>> reasons for RAID-3, especially if one is looking for consistent
>>> low-latency transactions like in video recorders and servers.
>>
>> Well, I did use *exactly* this example.  I also pointed out that the
>> relative performance of modern disk subsystems is adequate for a
>> single streaming video channel.
>>
>> Low latency depends on the number of concurrent accesses.  RAID-3
>> handles concurrent access poorly, exactly because it accesses all
>> disks for each transfer.
>
> One thing that RAID-3 has is that you never have to do a READ/MODIFY
> cycle when you do writes.  Until we implement a write-through cache
> geom module, raid-5 will continue to substandard performance.

Even then, RAID-5 might have higher bandwidth under some
circumstances.

My real question about RAID-3 remains: what use is it?  This isn't
nit-picking, it's certainly not a criticism of pjd.  I just don't see
any practical use on FreeBSD machines.

Greg
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