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

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