Re: [gentoo-user] mdadm and raid4

[Joost Roeleveld]

On Wednesday 04 May 2011 10:07:58 Evgeny Bushkov wrote:
> 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 it was superseded by RAID5.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.

I only use RAID-0 (when I want performance and don't care about the data), 
RAID-1 (for data I can't afford to loose) and RAID-5 (data I would like to 
keep). I have never bothered with RAID-4.

What do you see in the "dmesg" after the mdadm commands?
It might actually mention which is the parity disk in there.

--
Joost