From: | david(at)lang(dot)hm |
---|---|
To: | Mark Mielke <mark(at)mark(dot)mielke(dot)cc> |
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 23:36:38 |
Message-ID: | Pine.LNX.4.64.0712261534530.11785@asgard.lang.hm |
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Lists: | pgsql-performance |
On Wed, 26 Dec 2007, Mark Mielke wrote:
> 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.
yes, the two linux software implementations only read from one disk, but I
have seen hardware implementations where it reads from both drives, and if
they disagree it returns a read error rather then possibly invalid data
(it's up to the admin to figure out which drive is bad at that point).
no, I don't remember which card this was. I've been playing around with
things in this space for quite a while.
David Lang
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