[linux-lvm] Re: raid 1 on a single disk
clvm at spam.dragonhold.org
Sat Nov 13 21:13:24 UTC 2004
On Sat, Nov 13, 2004 at 04:11:08PM +0530, ashwin chaugule wrote:
> yes i know its silly ... im doing what im told to do :p
> ok so, i also do know, its performance is going to suck !
> but i was under the impression that RAID 1 works on more that one disks only.
> so you mean to say that, the linux RAID / md tools support raid 1 on
> multiple partitiions of the same disk ?
As people have already said, the software really doesn't care.
Here's a really sick/pointless example to prove it:
dd if=/dev/zero of=/var/tmp/fake1 bs=1M count=16
dd if=/dev/zero of=/var/tmp/fake2 bs=1M count=16
dd if=/dev/zero of=/var/tmp/fake3 bs=1M count=16
dd if=/dev/zero of=/var/tmp/fake4 bs=1M count=16
losetup /dev/loop1 /var/tmp/fake1
losetup /dev/loop2 /var/tmp/fake2
losetup /dev/loop3 /var/tmp/fake3
losetup /dev/loop4 /var/tmp/fake4
mdadm --create -l 5 /dev/md10 -n 4 /dev/loop1 /dev/loop2 /dev/loop3 /dev/loop4
md10 : active raid5 loop4 loop3 loop2 loop1
48960 blocks level 5, 64k chunk, algorithm 2 [4/3] [UUU_]
[================>....] recovery = 81.2% (14080/16320) finish=0.0min speed=1083K/sec
raidstop -c /dev/null /dev/md10
mdadm --create -l mirror /dev/md10 -n 4 /dev/loop1 /dev/loop2 /dev/loop3 /dev/loop4
md10 : active raid1 loop4 loop3 loop2 loop1
16320 blocks [4/4] [UUUU]
[=============>.......] resync = 68.7% (12160/16320) finish=0.0min speed=1105K/sec
What this has done is created a 48MB (roughly) raid5 set based on 4 16MB files in /var/tmp. Obviously this won't be recreated automatically on a reboot (since it's done using files), but it's to show what you can do if you are feeling sick enough. I've then created it as a mirror to show that works too.
Your command (to setup the mirror) will be something like:
mdadm --create -l mirror -n 4 /dev/hdd1 /dev/hdd2 /dev/hdd3 /dev/hdd4
And since thoswe are real disk partitions, if you set them to type "fd" in fdisk, they will be re-attached on a reboot.
P.S. I'd be really tempted to talk to the person asking you to do this, and ask them what they hope to achieve...
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