Re: Aligning things for SSDs

[tg%gmplib.org@localhost (Torbjörn Granlund)]

Martin Husemann <martin%duskware.de@localhost> writes:

  On Tue, Feb 17, 2015 at 10:12:54PM +0100, Torbjörn Granlund wrote:
  > If I use the install-time partitioning tool and use the default MB
  > sizes, things certainly seem to become poorly aligned; at least the
  > start sector numbers are not divisible by 8 (the 8 here comes from 4096
  > / 512).   Having the / partition at sector 63 starts off badly...
  
  Which version are you trying to install? The netbsd-7 beta should handle
  this correctly.
  
I am using a netbsd-7 from a few days ago.
  
What exactly should netbsd-7 handle correctly?

I made some experiements.  The partitioning tool step which allows
changes will show 'end' as the last sector + 1.  But the table in the
next step have them rounded down to the expected values.

I got aligned partitions to (appear to) work, except that aligning / to
64 instead of 63 caused non-boot.  (I assume the boot loader use a
hardwired offset.)

This was a bit trickier than one would want it to be.  A small mistake
could cause things to be unaligned again.  My final label looks good:

#        size    offset     fstype [fsize bsize cpg/sgs]
 a:    524225        63     4.2BSD   1024  8192     0
 c: 468862065        63     unused      0     0      
 d: 468862128         0     unused      0     0      
 e:  12582912    524288     4.2BSD   2048 16384     0
 f:   6291456  13107200     4.2BSD   2048 16384     0
 g: 449463472  19398656     4.2BSD   4096 32768     0

Unless I am much mistaken, almost every NetBSD install on an SSD will
end up aligning things very poorly, since the odd offset 63 will
propagate when sizes are given in MB.  I think such non-alignment hurt
spinning rust too.

-- 
Torbjörn
Please encrypt, key id 0xC8601622