New BSD Installer

Freddie Cash fjwcash at
Fri Feb 17 15:21:46 UTC 2012

On Fri, Feb 17, 2012 at 3:12 AM, Pete French <petefrench at> 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

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