On 11/13/12 06:30, Polytropon wrote:
On Mon, 12 Nov 2012 22:14:11 -0800, Ronald F. Guilmette wrote:
Which "partitions" need to be aligned to the 4KB boundaries?
The FreeBSD ones, the MBR ones, or both?

The partitions, all of them. :-)

For MBR partitions, the "DOS primary partitions", which are
slices, you typically only need one if you want to stay in
compatibility mode. For dedicated mode, you don't need it.

The slice typically starts in sector 63 and occupies the
space until the end of the device.

The partitions within the slice should have sizes of
multiples of 1 MB or 1 GB (which makes them multiples
of 4k easily).


Even though the handbook elaborates on the GPT approach, it
will work with traditional disklabel partitioning too.

Note that 4k = 8 x 512 byte, and so 64 sectors would be a
good alignment "grid", while 63 sectors is not. That implies
that in case you use fdisk to create a slice holding your
partitions, try to make it start at sector 64 (63 would
have been the default).

After that, use bsdlabel to create the partitions inside
the slice as you want. Make them multiples of 1M or 1G,
that should be no big deal because disks are big and cheap
today. :-)

You can then easily use newfs with the -f parameter:

        newfs -U -f 4096 <device>

This will make sure the proper fragment size will be applied
upon formatting the created partitions.

Also see:

As I have limited experience, anyone having more practical
experience with this matter is welcome to comment. :-)

According to the manual as of 9.0-RELEASE the default fragment and block sizes for newfs are 4k and 32k, so provided your partitions/slices are 4k aligned everything Should Just Work. Before 9.0 fragments and blocks were 2k and 16k which doesn't play so well with 4k drives.

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