On 04.05.2011 01:49, Florian Philipp wrote:
> Am 03.05.2011 19:54, schrieb Evgeny Bushkov:
>> Hi.
>> How can I find out which is the parity disk in a RAID-4 soft array? I
>> couldn't find that in the mdadm manual. I know that RAID-4 features a
>> dedicated parity disk that is usually the bottleneck of the array, so
>> that disk must be as fast as possible. It seems useful to employ a few
>> slow disks with a relatively fast disk in such a RAID-4 array.
>>
>> Best regards,
>> Bushkov E.
>>
> You are seriously considering a RAID4? You know, there is a reason why
> it was superseded by RAID5. Given the way RAID4 operates, a first guess
> for finding the parity disk in a running array would be the one with the
> worst SMART data. It is the parity disk that dies the soonest.
>
> From looking at the source code it seems like the last specified disk is
> parity. Disclaimer: I'm no kernel hacker and I have only inspected the
> code, not tried to understand the whole MD subsystem.
>
> Regards,
> Florian Philipp
>
Thank you for answering... The reason I consider RAID-4 is a few
sata/150 drives and a pair of sata II drives I've got. Let's look at
the problem from the other side: I can create RAID-0(from sata II
drives) and then add it to RAID-4 as the parity disk. It doesn't bother
me if any disk from the RAID-0 fails, that wouldn't disrupt my RAID-4
array. For example:
mdadm --create /dev/md1 --level=4 -n 3 -c 128 /dev/sdb1 /dev/sdc1 missing
mdadm --create /dev/md2 --level=0 -n 2 -c 128 /dev/sda1 /dev/sdd1
mdadm /dev/md1 --add /dev/md2
livecd ~ # cat /proc/mdstat
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md2 : active raid0 sdd1[1] sda1[0]
20969472 blocks super 1.2 128k chunks
md1 : active raid4 md2[3] sdc1[1] sdb1[0]
20969216 blocks super 1.2 level 4, 128k chunk, algorithm 0 [3/2] [UU_]
[========>............] recovery = 43.7% (4590464/10484608)
finish=1.4min speed=69615K/sec
That configuration works well, but I'm not sure if md1 is the parity
disk here, that's why I asked. May be I'm wrong and RAID-5 is the only
worth array, I'm just trying to consider all pros and cons here.
Best regards,
Bushkov E.
