Re: misunderstanding of spare and raid devices?

[Phil Turmel <philip@turmel.org>]

Hi Karsten,

On 06/30/2011 07:30 AM, John Robinson wrote:
> On 30/06/2011 11:51, Karsten Römke wrote:
>> Hello,
>> I'm searching some hours / minutes to create a raid5 device with 4 disks
>> and 1 spare:
>> I tried first with the opensuse tool but no success as I want, so I
>> tried mdadm
>>
>> Try:
>> mdadm --create /dev/md0 --level=5 --raid-devices=4 --spare-devices=1
>> /dev/sda3 /dev/sdb2 /dev/sdc5 /dev/sdd5 /dev/sde5
>>
>> leads to
>> Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
>> md0 : active (auto-read-only) raid5 sdd5[5](S) sde5[4](S) sdc5[2]
>> sdb2[1] sda3[0]
>> 13759296 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
>>
>> 2 spares - I don't understand that.

Just to clarify for you, as your comment below suggests some confusion as to the role of a spare:

When the resync finished on this, if you had let it, you would have had three drives' capacity, with parity interspersed, on four drives.  The fifth drive would have been idle, but ready to replace any of the other four without intervention from you.

>> kspace9:~ # mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda3
>> /dev/sdb2 /dev/sdc5 /dev/sdd5
>> leads to
>> md0 : active (auto-read-only) raid5 sdd5[4](S) sdc5[2] sdb2[1] sda3[0]
>> 13759296 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
>>
>> 1 spare - but why - I expect 4 active disks and 1 spare
>>
>> kspace9:~ # mdadm --create /dev/md0 --level=5 --raid-devices=5 /dev/sda3
>> /dev/sdb2 /dev/sdc5 /dev/sdd5 /dev/sde5
>> leads to
>> md0 : active (auto-read-only) raid5 sde5[5](S) sdd5[3] sdc5[2] sdb2[1]
>> sda3[0]
>> 18345728 blocks level 5, 64k chunk, algorithm 2 [5/4] [UUUU_]

This will end up with four drives' capacity, with parity interspersed, on five drives.  No spare.

>> That's what I want, but I reached it more or less by random.
>> Where is my "think-error" (in german).

I hope this helps you decide which layout is the one you really want.  If you think you want the first layout, you should also consider raid6 (dual redundancy).  There's a performance penalty, but your data would be significantly safer.

Phil
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html