Re: [gentoo-user] mdadm and raid4

[Evgeny Bushkov <zhen@dotcomltd.ru>]

On 04.05.2011 13:38, Joost Roeleveld wrote:
> 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 it was superseded by RAID5.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...
>
> --
> Joostlivecd ~ # mdadm --misc --detail /dev/md1
livecd ~ # mdadm --misc --detail /dev/md1
/dev/md1:
        Version : 1.2
  Creation Time : Wed May  4 13:54:33 2011
     Raid Level : raid4
     Array Size : 122624 (119.77 MiB 125.57 MB)
  Used Dev Size : 61312 (59.89 MiB 62.78 MB)
   Raid Devices : 3
  Total Devices : 3
    Persistence : Superblock is persistent

    Update Time : Wed May  4 13:55:14 2011
          State : clean
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

     Chunk Size : 128K

           Name : livecd:1  (local to host livecd)
           UUID : 654218f0:9f0b88d5:d82f39bc:ae08aa1e
         Events : 19

    Number   Major   Minor   RaidDevice State
       0       8       17        0      active sync   /dev/sdb1
       1       8       33        1      active sync   /dev/sdc1
       3       9        2        2      active sync   /dev/md2

Best regards,
Bushkov E.