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 it was superseded by RAID5.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
