Re: RAID 5 vs RAID 10 benchmark

From: Frank van Bortel <fvanbortel_at_netscape.net>
Date: Sat, 27 Nov 2004 13:01:58 +0100
Message-ID: <co760g$kne$1@news5.zwoll1.ov.home.nl>

Howard J. Rogers wrote:
> Jan wrote:
>

>> Several scientific (or logical) conclussions were already made about
>> RAID 5
>> that this is not the best option for write intensive databases. It
>> seems very logical also to me when I read arguments against RAID 5.
>>
>> But I`m confused by following benchmark showing that it is not
>> worthful anymore (at least for a tested storage) to thinks about RAID
>> 5 vs RAID 10 from the performance point of view.
>>
>> http://www-1.ibm.com/servers/storage/disk/ess/pdf/raid5-raid10.pdf
>>
>> Jan

>
>
> The concern with RAID 5 is not whether there is a write penalty so much.
> With these days of massive battery-backed cache, that is almost a
> non-event (though the "I" in RAID has, as a result, long since ceased to
> mean 'inexpensive').
>
> The real concern with RAID 5 is what happens when you pull one of the
> disks out. A single physical I/O now requires a read to be made against
> every surviving member of the array, so that the missing data can be
> deduced. RAID5's I/O performance under failure conditions is woeful. And
> you suffer it for the duration of the array rebuild when you plug the
> disk back in.
>
> Try the same thing with RAID 1+0... the loss of one disk makes no
> difference, since a complete array is still available. And plugging the
> disk back in means there's just one disk to re-silver from.
>
> Regards
> HJR
In addition to that:
- pull out a second one on RAID-5: nothing goes. - pull out a second one on RAID 1+0: who cares?

    (unless you happen to pull the one disk, that      had no mirror anymore)

Basic message: RAID10 (or 1+0) is *safer* than RAID5, because you can *never* loose 2 disks in RAID5 without serious disruption (involving restoring of backups).

-- 

Regards,
Frank van Bortel

Received on Sat Nov 27 2004 - 06:01:58 CST