New BSD Installer
wblock at wonkity.com
Fri Feb 17 15:49:44 UTC 2012
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.
> 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.
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