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Re: Tyan Thunder MB for postgres server

From: William Yu <wyu(at)talisys(dot)com>
To: Iain <iain(at)mst(dot)co(dot)jp>
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: Tyan Thunder MB for postgres server
Date: 2004-12-16 07:14:03
Message-ID: 41C135BB.3030702@talisys.com (view raw or flat)
Thread:
Lists: pgsql-admin
Iain wrote:
> Hi William,
> 
>> SOmething to think about. Let's suppose a channel/cable completely 
>> dies. How would you protect against it? Split a logical mirror device 
>> over 2 channels.
> 
> 
> This effectively implements RAID 0+1, right? RAID 1 (mirroring) over 
> RAID 0 striped volumes. I can certainly see your point regarding the 
> redundancy of the controller channels, but my understanding is that 
> (apart from that) RAID 0+1 is less robust that RAID 10 regarding disk 
> failures. Presuming that the system will continue to operate even in the 
> event of 1 channel failure, it's still not a clear choice. Does that 
> seem like a reasonable assessment?

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

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


>> Another trick I've started doing with my MegaRAID setups is mirroring 
>> in hardware but striping in software.
> 
> 
> Yeah, that is a good point. I havn't decided either way but I consider 
> that a viable option.

You can probably do the same hardware + software trick for RAID10. 
Except after OS crashes, Linux+Solaris spends quite a bit of time 
resyncing mirrors and it looks like the MegaRAID controller is a bit 
smarter in knowing when to resync.


> If you were building this system now, and want the option of buying the 
> same disks in 3 years time, do you think it would be a bad idea to go 
> for the ~40GB size? Maybe the next size up would be better, though we 
> don't actually need the extra space.

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.


> Also, someone asked me what happens if one of the CPUs fails on this 
> system, will the system continue to operate on 1 CPU. I havn't really 
> considered this, and have never read anything either way, so my 
> assumption is "no, it won't". Any comment?

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

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