Hi,
On Thu, 1 Jun 2000, John Saunders wrote:
> I have a RH6.1 box with two mirrored drives in a RAID1 config.
>
> I am wanting to add a spare disk and would like advice on the correct
> procedure for adding it to the existing config.
With RAID1 there is little point in adding a "spare disk" - you can just
add another disk to the running array, ie:
md0: sda1 sdb1 sdc1 [RAID1]
Means that partitions sdb1 and sdc1 /always/ contains an exact copy of
sda1. This array can survive the failure of any two devices.
The way I have configured my server is:
md0: sda1 sdb1 sdc1 [RAID1] /
md1: sda2 sdb2 sdc2 sdd2 [RAID5] /home
(sdd1 is a "cold" mirror of sda1)
This means that everything will work under a one-disk failure, and the
root partition will still work under a two-disk failure.
The reasons that adding these disks to the RAID1 mirror is better than
having it as a spare are:
i) Performance will increase with Mika's read-balancing patch (great!)
ii) When a disk fails in the case of the 'spare', the data has to be
copied over to the it. This will impair performance for a short time,
and means that if there is another failure before the duplication is
completed, then the array will go down.
> What I planned was:
>
> 1. Bring the system down.
> 2. Install the spare drive
> 3. Boot up and edit the /etc/raidtab to include the spare drive
> 4. fdisk the drive to be identical to the mirror'd drives
> 5. raidhotadd the drive
>
> Is this correct and will the drive then be added automatically should a
> drive fail and the system is rebooted ?.
AFAIK this will work perfectly.
If you decide to go with adding it to the array proper instead of as a
spare disk, then just change that in the raidtab.
As an aside, I was wondering if there might ever be support for having a
spare disk assigned to more than one array? ie you could assign a pool of
spare disks to all arrays, which would be removed as spare disks if used
in the event of a failure... just a thought :)
Hope this helps!
Corin
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