On Mon, 27 Aug 2012, Kevin Oberman wrote:
On Mon, Aug 27, 2012 at 1:16 PM, Warren Block <[email protected]> wrote:
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.
You can force alignment by use of -b. I just managed to miss that you
were doing that. '-a' simply does the alignment and I have no reason
to forces the location of any partition as all are multiples of 1M and
4K. Use of -a and -b on the same command seems rather useless,
Might make more sense if -a is seen as a safety check. And yes, -b is
an exception, done in this case to get the first partition at a specific
spot.
but it seems that ignoring -b is still a bug.
Works for me in 9-stable. Here's the change in -head:
http://svnweb.freebsd.org/base/head/sbin/geom/class/part/geom_part.c?r1=229916&r2=235033
It was MFCed to 8-stable and 9-stable on 2012-05-11.
I'm not sure I get your statement that "All that -a does is round
partition starting blocks and sizes to even values. " -a aligns the
start of every partition to the stated size (as your example showed).
Sorry, I should have been more precise with the wording. By "even" I
meant even multiple of the given block value.
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.
True, but so is 4K (8-512 byte blocks). Obviously 1M is also a
multiple of all powers of 2 below it as is 4K. Even in this age of
cheap disks, 1G alignment seems a bit extreme, but in most cases, it
Er, 1M. It leaves a little less than 512K of unused space. Starting at
1G would be a more difficult decision for me, though you're right that
it's a trivial amount of space on a lot of computers.
really is insignificant for general purpose systems. It is an argument
for single partitions, but I always worry that something screwy will
blow up /var with log messages and I would not want this to fill all
disk space, so I like to keep that, as well as a read-only root. Just
old-fashioned, I guess.
Understood. Usually separate filesystems for me, although I recently
took to using tmpfs for /tmp.
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