RAID-3?
Scott Long
scottl at samsco.org
Thu Aug 19 00:05:54 PDT 2004
Greg 'groggy' Lehey wrote:
> On Thursday, 19 August 2004 at 8:33:58 +0200, Poul-Henning Kamp wrote:
>
>>In message <20040819062228.GO85432 at wantadilla.lemis.com>, "Greg 'groggy' Lehey"
>> writes:
>>
>>>On Thursday, 19 August 2004 at 0:00:55 -0600, Scott Long wrote:
>>>
>>>>I think that you're really reading far too much into this.
>>>
>>>That depends on whether you care about accurate terminology or not.
>>>Or maybe it's you who is reading too much into the matter.
>>
>>I think being accurate is a great thing, but accuracy of definition
>>should never get in the way of working code.
>
>
> Agreed. I don't think it is.
>
>
>>The main features of RAID3 are the always full stripe access which
>>keeps your disk heads running in tandem which has desirable
>>performance characteristica.
>
>
> ... for single accessors.
>
> But a single IDE drive nowadays can transfer 40 MB a second. A 5 disk
> RAID-3 array should thus be able to transfer 160 MB a second. What do
> you need that for?
>
Video streaming and recoding would find this quite useful I would think.
But regardless, it's not about thoroughput, it's about having
predictable latency. I can't stress this enough!
>
>>Also the fact that you can trivially add ECC instead of mere parity
>>is a big plus.
>
>
> Ah, but that would be RAID-2. Or something similar.
>
>
>>Raid5 with two bit ECC (sometimes called raid6)
>
>
> I thought RAID-6 was RAID-5 with two identical parity disks. Not so?
>
>
>>is a royal nightmare to code (see the raidframe paper)
>
>
> Does this define RAID-6, or just describe the pain?
>
There is no formal definition of RAID-6. There are various competing
companies that have tried to position their products as the de-facto
RAID-6, but that isn't terribly useful here.
>
>>whereas RAID3 in 4+2 or 8+3 is pretty trivial because of the
>>full-stripe access pattern.
>
>
> Sure, easy coding is good. And having written a RAID-5
> implementation, I can believe what a nightmare that an ECC version
> might provide.
>
Ah, but that is the simplicity of RAID-3. Your ECC/FEC/Parity
calculation is relatively easy and deterministic to code since you are
always writing to all disks at the same time.
I'll concede that a general-purpose PC has challenges in meeting the
strict interpretation of RAID-3, but what Pawel has meets enough of
the common definition that I think that it's Close Enough and the
vast majority of users will get what they expect from it.
Scott
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