On Wednesday 04 May 2011 13:08:34 Evgeny Bushkov wrote: > 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 it was superseded by RAID5.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.
What's the result of: mdadm --misc --detail /dev/md1 ? Not sure what info this command will provide with a RAID-4... -- Joost
