Re: ASM LUN sizes and number of disks

On Fri, Nov 7, 2008 at 6:55 PM, Finn Jorgensen <finn.oracledba_at_gmail.com> wrote:
> Because of the way ASM distributes data evenly across all disks in a
> diskgroup, adding 200GB disks to an existing diskgroup comprising of 50GB
> disks means you will never be able to use more than 50GB of those 200GB
> disks.

I agree with this...

> You will have to add those disks to a separate diskgroup and then start
> moving data over, which means downtime.

but could you not add(2x200)/rebalance, drop(6x50GB)/rebalance to get all the data onto the 2x200GB?
this would alleviate downtime but...

I would never put my whole database only on 2 spindles anyway. If I recall correctly, if there is not room to mirror ASM extents from the failed drive, the diskgroup will dismount to protect from catastrophic failure. This means in a two disk ASM group, losing one disk will result in a down database. There there is also a special case with 3 disks, as each is in their own failgroup and I believe the usable space (for files) for 3 disks is the same as 2 disks (or something similar). So basically the minimum recommended disks is four. It comes back to disk space is cheap...

-- 
Regards,
Greg Rahn
http://structureddata.org
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