> With RAID1 there is little point in adding a "spare disk" - you can just
> add another disk to the running array, ie:
You cannot "add" a third to to a two-way RAID1.
You can create a new three-way RAID1 if you want.
But you can't (AFAIK) change the number of disk is the base array.
> md0: sda1 sdb1 sdc1 [RAID1]
This means that every write has to be made to three disks rather than two.
If IO bandwidth is a concern, this is not a good thing.
> Means that partitions sdb1 and sdc1 /always/ contains an exact copy of sda1.
Not quite *always* -- only until there is a fault !
> This array can survive the failure of any two devices.
Indeed -- a "2-way + spare" cannot survive two failures in less than the time
it takes to resync the spare. SO if that matters to you, and you can afford
the BW, use 3-way -- it may even speed up reading (which can RAID0).
> i) Performance will increase with Mika's read-balancing patch (great!)
Indeed -- if doing mostly reading, this is good.
> 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.
Agreed, but having to do triple rather than double writes may `impair
performance' all the time. It depends what matters in your environment.
>> 3. Boot up and edit the /etc/raidtab to include the spare drive
>> 5. raidhotadd the drive
> 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.
No -- that won't work (at least not with my tools, if you have PSB, which
I assume you do!).
To enlarge an array, what you should do is to create a new raid entry with the
max number of disks you want, and have all but the N which are already valid
marked in raidtab as raid-failed.
Then run mkraid. This will sync the N which are already valid, which should
work (assuming they are already in sync).
Then `raid hot add' the other (not pre-sync'ed) ones.
> As an aside, I was wondering if there might ever be support for having a
> spare disk assigned to more than one array?
I've long wanted this -- it seems silly (but easier to implement) to have to
have N spare disks for N arrays ...
