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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