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Re: Tyan Thunder MB for postgres server

From: "Iain" <iain(at)mst(dot)co(dot)jp>
To: "William Yu" <wyu(at)talisys(dot)com>
Cc: <pgsql-admin(at)postgresql(dot)org>
Subject: Re: Tyan Thunder MB for postgres server
Date: 2004-12-16 08:38:39
Message-ID: 000901c4e34a$a6770700$7201a8c0@mst1x5r347kymb (view raw, whole thread or download thread mbox)
Lists: pgsql-admin
Thanks again for your valuable input William.

> I'm not sure why people say one is better than the other. Both will 
> survive the loss of 2 drives -- they're just different drives.

> RAID 0+1: A(1m1) s B(1m1) <-- any drive on A and any drive on B
> RAID 10:  A(1s1) m B(1s1) <-- both drives on A or both drives on B

I'm wondering if we have a terminology problem here. As I understand it the 
configs are the reverse of what you said. I only checked 
so I hope they're not wrong! I quote:

"RAID 0+1 is implemented as a mirrored array whose segments are RAID 0"

"RAID 10 is implemented as a striped array whose segments are RAID 1"

This is the quote that interested me "RAID 0+1 is NOT to be confused with 
RAID 10. A single drive failure will cause the whole array to become, in 
essence, a RAID Level 0 array" and "Under certain circumstances, RAID 10 
array can sustain multiple simultaneous drive failures".

As bytepile has it, failure of 1 disk in 0+1 leaves you with just RAID 0 so 
one more failure on the other pair and your data is gone. On the other hand, 
failure of 1 disk in raid 10 leaves you with a working raid 1 that can 
sustain a second failure.

> Either way, I think you can do both across 2 channels to be redundant for 
> a channel/cable failing.
> RAID 0+1: C1-m-C2 s C1-m-C2
> RAID 10:  C1-s-C1 m C2-s-C2

Gaaa! I'll have to ponder that one. C1-m-C2 s C1-m-C2 sounds good in theory. 
Presumably to be practical it would require hardware and maybe software 
support, so the machine continues to operate despite the failure (and 
hopefully tells you about it). It would interesting to test it anyway.

> Always get more space than you need if you can afford it because it's a 
> pain in the ass adding more disk space. Especially on the boot drives.

Very true.

> Doing a google search for "hot swap cpu", I see they've added something 
> into Linux but you'd still need hardware support.

I'm sure that if the Tyan board had that support they'd be making a fuss of 
it :-) I'll have a look for "hot swap cpu" too though, just for interests 

High redundancy is becoming more accessible I think, but it still takes a 
lot more money than your average PC server motherboard costs to get it ;-) 
something to look forward to anyway.


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