Re: [gentoo-user] mdadm and raid4

[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 it was superseded by RAID5.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.
>
> 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.