create a partial raid5

create a partial raid5

[stephan sperber]

Hello!

Is it possible to create a Radi5 with only 2 disks? (I know that Raid5 
needs 3 to be really redundant)

With a Raid1 I can do things like:
mdadm -C /dev/md0 --force --raid-devices=1 /dev/hdb2
mkreiserfs /dev/md0

copy files from an older drive to the raid, get the old drive out and put 
the new disk for the raid in.
then
mdadm -G /dev/md0 --raid-disks=2
mdadm -a /dev/md0 /dev/hdc2

and the raid1 is now a real raid1 (after sync)

It would be great to have something similar for raid5:
mdadm -C /dev/md0 --force --raid-devices=2 /dev/hdb2 /dev/hdc2
mdadm -G /dev/md0 --raid-disks=3
mdadm -a /dev/md0 /dev/hda2

or
mdadm -C /dev/md0 --force --fail=/dev/hda2 --raid-devices=3 /dev/hdb2 /
dev/hdc2
mdadm -a /dev/md0 /dev/hda2

i know that this is possible with raidtools and /etc/raidtab.

is this possible with the recent version of mdadm or is it a planned 
feature (or not planned)?

thanks in advance
stephan



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Re: create a partial raid5

[Troels Bang Jensen]

Oh yeah, it's possible - I've done it in connection with disaster  
recovery (or rather, attempt thereof).

The key is to write 'missing' instead of the device name, so you just go

mdadm -C /dev/md0 --raid-level=5 --raid-devices=3 /dev/hdxx /dev/hdyy  
missing

-note that the missing drive can be anywhere you like.

Regards, Troels


On Feb 9, 2006, at 12:40 , stephan sperber wrote:

> Hello!
>
> Is it possible to create a Radi5 with only 2 disks? (I know that Raid5
> needs 3 to be really redundant)
>
> With a Raid1 I can do things like:
> mdadm -C /dev/md0 --force --raid-devices=1 /dev/hdb2
> mkreiserfs /dev/md0
>
> copy files from an older drive to the raid, get the old drive out  
> and put
> the new disk for the raid in.
> then
> mdadm -G /dev/md0 --raid-disks=2
> mdadm -a /dev/md0 /dev/hdc2
>
> and the raid1 is now a real raid1 (after sync)
>
> It would be great to have something similar for raid5:
> mdadm -C /dev/md0 --force --raid-devices=2 /dev/hdb2 /dev/hdc2
> mdadm -G /dev/md0 --raid-disks=3
> mdadm -a /dev/md0 /dev/hda2
>
> or
> mdadm -C /dev/md0 --force --fail=/dev/hda2 --raid-devices=3 /dev/ 
> hdb2 /
> dev/hdc2
> mdadm -a /dev/md0 /dev/hda2
>
> i know that this is possible with raidtools and /etc/raidtab.
>
> is this possible with the recent version of mdadm or is it a planned
> feature (or not planned)?
>
> thanks in advance
> stephan
>
>
>
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> NEU: GMX Phone_Flat http://www.gmx.net/de/go/telefonie
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