Am 04.05.2011 14:39, schrieb Florian Philipp: > Am 04.05.2011 11:08, schrieb Evgeny Bushkov: >> 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 it was superseded by RAID5.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. >>> > [...] >> >> 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. >> >> >> > > When you have an array with uneven disk speeds, you might consider using > the --write-mostly option of mdadm: > -W, --write-mostly > subsequent devices lists in a --build, --create, or --add command > will be flagged as 'write-mostly'. This is valid for RAID1 only and > means that the 'md' driver will avoid reading from these devices if at > all possible. This can be useful if mirroring over a slow link. > > This should help in concurrent read and write operations because the > kernel will not dispatch read requests to a disk that is already having > trouble managing the write operations. > > On another point: Are you sure your disks have different speeds? SATA150 > and 300 are no reliable indicator because most HDDs cannot saturate the > SATA port anyway. dd is still the most reliable way to measure > sequential throughput. > > Regards, > Florian Philipp >
`man 4 md` also states that the "the last of the active devices in the array" is the parity disk in a RAID4. Regards, Florian Philipp
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