Floating Spare

Floating Spare

[SoulBlazer]

Can you share 2 or 3 'spare-disks' between multiple raid sets?

Eg. I have a 22 disk array chopped into multiple 4 disk raid-5 volumes, this 
leaves me with 2 disks left over..  Can I assign these two drives as floating 
spares incase of failure.. this will obviously be a 'first come first serve' 
scenario for raid failure.. but its better then not using them.

## Vol 00 ##
raiddev /dev/md/0
        raid-level              5
        nr-raid-disks           3
        nr-spare-disks          1
        persistent-superblock   1
        chunk-size              128
        parity-algorithm        left-symmetric

        device          /dev/scsi/host2/bus0/target0/lun0/part1
        raid-disk       0
        device          /dev/scsi/host2/bus0/target1/lun0/part1
        raid-disk       1
        device          /dev/scsi/host2/bus0/target2/lun0/part1
        raid-disk       2
        device          /dev/scsi/host2/bus0/target3/lun0/part1
        spare-disk      0

## Vol 01 ##
raiddev /dev/md/1
        raid-level              5
        nr-raid-disks           3
        nr-spare-disks          1
        persistent-superblock   1
        chunk-size              128
        parity-algorithm        left-symmetric

        device          /dev/scsi/host2/bus0/target4/lun0/part1
        raid-disk       0
        device          /dev/scsi/host2/bus0/target5/lun0/part1
        raid-disk       1
        device          /dev/scsi/host2/bus0/target6/lun0/part1
        raid-disk       2
        device          /dev/scsi/host2/bus0/target3/lun0/part1
        spare-disk      0

Re: Floating Spare

[Marcel]

SoulBlazer wrote:
> Can you share 2 or 3 'spare-disks' between multiple raid sets?
> 
> Eg. I have a 22 disk array chopped into multiple 4 disk raid-5 volumes, this 
> leaves me with 2 disks left over..  Can I assign these two drives as floating 
> spares incase of failure.. this will obviously be a 'first come first serve' 
> scenario for raid failure.. but its better then not using them.

I'm sure you could script something to that effect based on the current 
RAID tools and their options/output...

If there's nothing like it already included (I haven't checked - and no 
in-depth knowledge of raid tools either, so forgive any stupid remarks) 
I could imagine a cron job checking the logs for disk failures and 
hot-adding the "new" disk (one of the spares, if available).

Interesting idea!

Marcel

Re: Floating Spare

[Neil Brown]

On Monday April 8, soul@lamp-post.net wrote:
> Can you share 2 or 3 'spare-disks' between multiple raid sets?
> 
> Eg. I have a 22 disk array chopped into multiple 4 disk raid-5 volumes, this 
> leaves me with 2 disks left over..  Can I assign these two drives as floating 
> spares incase of failure.. this will obviously be a 'first come first serve' 
> scenario for raid failure.. but its better then not using them.

The kernel driver doesn't directly support this.
However "mdadm" does.
Get  mdadm from
   http://www.cse.unsw.edu.au/~neilb/source/mdadm/

compile, and read the doco.

You describe your arrays in /etc/mdadm.conf and mention that they are
both in the same "spare-group", and then run "mdadm --monitor".
This periodically polls the arrays, emails you when there are
failures, and will move spares between arrays if needed.

NeilBrown

/etc/mdadm.conf:
 
  DEVICES /dev/disc/*/*
  array /dev/md/0 uuid=whatever spare-group=A
  array /dev/md/1 uuid=somethingelse spare-group=A

You only need "uuid=" if you want to use mdadm to assemble your arrays
aswell.

NeilBrown