New BSD Installer

[Warren Block]

On Fri, 17 Feb 2012, Freddie Cash wrote:

> 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.

Potentially, yes.

> And the more partitions you have, the worse it gets.

One big mirrored partition avoids it, but then there's only one 
partition.  (ad0p2a?  Forget I mentioned that.)

> 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.

Some queuing logic in the mirror rebuild could avoid that.  I am 
blissfully unaware of how complicated that might be.