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Re: With 4 disks should I go for RAID 5 or RAID 10

From: Mark Mielke <mark(at)mark(dot)mielke(dot)cc>
To: david(at)lang(dot)hm
Cc: Florian Weimer <fw(at)deneb(dot)enyo(dot)de>, Fernando Hevia <fhevia(at)ip-tel(dot)com(dot)ar>, 'pgsql-performance' <pgsql-performance(at)postgresql(dot)org>
Subject: Re: With 4 disks should I go for RAID 5 or RAID 10
Date: 2007-12-26 21:57:08
Message-ID: 4772CE34.2040608@mark.mielke.cc (view raw or flat)
Thread:
Lists: pgsql-performance
david(at)lang(dot)hm wrote:
> On Wed, 26 Dec 2007, Mark Mielke wrote:
>
>> Florian Weimer wrote:
>>>> seek/read/calculate/seek/write since the drive moves on after the
>>>> read), when you read you must read _all_ drives in the set to check
>>>> the data integrity.
>>> I don't know of any RAID implementation that performs consistency
>>> checking on each read operation. 8-(
>> Dave had too much egg nog... :-)
>> Yep - checking consistency on read would eliminate the performance 
>> benefits of RAID under any redundant configuration.
> except for raid0, raid is primarily a reliability benifit, any 
> performance benifit is incidental, not the primary purpose.
> that said, I have heard of raid1 setups where it only reads off of one 
> of the drives, but I have not heard of higher raid levels doing so.
What do you mean "heard of"? Which raid system do you know of that reads 
all drives for RAID 1?

Linux dmraid reads off ONLY the first. Linux mdadm reads off the "best" 
one. Neither read from both. Why should it need to read from both? What 
will it do if the consistency check fails? It's not like it can tell 
which disk is the right one. It only knows that the whole array is 
inconsistent. Until it gets an actual hardware failure (read error, 
write error), it doesn't know which disk is wrong.

Cheers,
mark

-- 
Mark Mielke <mark(at)mielke(dot)cc>

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