9.1 RELENG_9 Unable to cleanly dismount root partition on shutdown

[Warren Block]

On Mon, 27 Aug 2012, Kevin Oberman wrote:

>> No obvious problems jumped out at me.  Here are my notes:
>> http://www.wonkity.com/~wblock/docs/html/disksetup.html
>>
>> The gpart version is halfway down.  I really need to switch that around.
>
> Pretty good page, but I would really suggest that you also do either
> 4k or 1M alignment on your partitions. If you don't and use a disk
> with 4K blocks (internally), you will have terrible performance.

You mean add the -a parameter for gpart?  All that -a does is round 
partition starting blocks and sizes to even values.  If the numbers 
given are already even multiples, it does nothing.

The reason -a4k is not shown there is because until a few months ago, -a 
overrode -b.  So

   # gpart add -t freebsd-ufs -l gprootfs -a4k -b 1M -s 2G da0

did not start that partition at 1M, but instead at the next even 4K 
block after the first 512K partition; block 1064 instead of block 2048, 
AFAIR.  The fix to gpart (thanks to ae@) is in 9-stable and 9.1, but not 
earlier releases.

Mentioned a little farther down in the article is that keeping 
additional partitions to even multiples of 1M or 1G size will keep them 
in alignment.

> 1M is recommended by Microsoft and used by Windows, but seems a bit 
> excessive to me.

Also by some Sun RAID controllers and other systems.  1M is a nice even 
multiple of a lot of common block sizes.