New BSD Installer

[Freddie Cash]

On Fri, Feb 17, 2012 at 3:12 AM, Pete French <petefrench at ingresso.co.uk> wrote:
>> I wasn't aware you could do that.  I was only aware that it was the
>> other way around.  That (my) misconception seems to also be relayed
>> by others such as Miroslav who said:
>
> Should this not be the recommended way of doing things even for MBR
> disks ? I have a lot of machines booting from gmirror, but we always
> do it by mirroring MBR partitions (or GPT ones). I cant see why you would
> want to do it the other way round in fact. It doesnt gain you anything
> does it ?

The problem with mirroring partitions is that you thrash the disk
during the rebuild after replacing a failed disk.  And the more
partitions you have, the worse it gets.

If you mirror the device, then the rebuild process only has to rebuild
a single "thing".

If you mirror 4 partitions on a device, then there will be four
simultaneous, parallel rebuild processes running, thrashing the drive
heads on both devices, killing you I/O throughput and extending the
length of the rebuild.

And if you mix your redundancy technologies (like gmirror and zfs
mirror) it gets even worse due to competing rebuild schedulers.

-- 
Freddie Cash
fjwcash at gmail.com