| 1 |
On 04.05.2011 01:49, Florian Philipp wrote: |
| 2 |
> Am 03.05.2011 19:54, schrieb Evgeny Bushkov: |
| 3 |
>> Hi. |
| 4 |
>> How can I find out which is the parity disk in a RAID-4 soft array? I |
| 5 |
>> couldn't find that in the mdadm manual. I know that RAID-4 features a |
| 6 |
>> dedicated parity disk that is usually the bottleneck of the array, so |
| 7 |
>> that disk must be as fast as possible. It seems useful to employ a few |
| 8 |
>> slow disks with a relatively fast disk in such a RAID-4 array. |
| 9 |
>> |
| 10 |
>> Best regards, |
| 11 |
>> Bushkov E. |
| 12 |
>> |
| 13 |
> You are seriously considering a RAID4? You know, there is a reason why |
| 14 |
> it was superseded by RAID5. Given the way RAID4 operates, a first guess |
| 15 |
> for finding the parity disk in a running array would be the one with the |
| 16 |
> worst SMART data. It is the parity disk that dies the soonest. |
| 17 |
> |
| 18 |
> From looking at the source code it seems like the last specified disk is |
| 19 |
> parity. Disclaimer: I'm no kernel hacker and I have only inspected the |
| 20 |
> code, not tried to understand the whole MD subsystem. |
| 21 |
> |
| 22 |
> Regards, |
| 23 |
> Florian Philipp |
| 24 |
> |
| 25 |
Thank you for answering... The reason I consider RAID-4 is a few |
| 26 |
sata/150 drives and a pair of sata II drives I've got. Let's look at |
| 27 |
the problem from the other side: I can create RAID-0(from sata II |
| 28 |
drives) and then add it to RAID-4 as the parity disk. It doesn't bother |
| 29 |
me if any disk from the RAID-0 fails, that wouldn't disrupt my RAID-4 |
| 30 |
array. For example: |
| 31 |
|
| 32 |
mdadm --create /dev/md1 --level=4 -n 3 -c 128 /dev/sdb1 /dev/sdc1 missing |
| 33 |
mdadm --create /dev/md2 --level=0 -n 2 -c 128 /dev/sda1 /dev/sdd1 |
| 34 |
mdadm /dev/md1 --add /dev/md2 |
| 35 |
|
| 36 |
livecd ~ # cat /proc/mdstat |
| 37 |
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10] |
| 38 |
md2 : active raid0 sdd1[1] sda1[0] |
| 39 |
20969472 blocks super 1.2 128k chunks |
| 40 |
|
| 41 |
md1 : active raid4 md2[3] sdc1[1] sdb1[0] |
| 42 |
20969216 blocks super 1.2 level 4, 128k chunk, algorithm 0 [3/2] [UU_] |
| 43 |
[========>............] recovery = 43.7% (4590464/10484608) |
| 44 |
finish=1.4min speed=69615K/sec |
| 45 |
|
| 46 |
That configuration works well, but I'm not sure if md1 is the parity |
| 47 |
disk here, that's why I asked. May be I'm wrong and RAID-5 is the only |
| 48 |
worth array, I'm just trying to consider all pros and cons here. |
| 49 |
|
| 50 |
Best regards, |
| 51 |
Bushkov E. |
Archives