On 04.05.2011 11:54, Joost Roeleveld wrote: > 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 it was superseded by RAID5.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 > There's nothing special in dmesg:
md: bind<md2> RAID conf printout: --- level:4 rd:3 wd:2 disk 0, o:1, dev:sdb1 disk 1, o:1, dev:sdc1 disk 2, o:1, dev:md2 md: recovery of RAID array md1 I've run some tests with different chunk sizes, the fastest was raid-10(4 disks), raid-5(3 disks) was closely after. Raid-4(4 disks) was almost as fast as raid-5 so I don't see any sense to use it. Best regards, Bushkov E.
