Partition alignment
Warren Block
wblock at wonkity.com
Wed Mar 26 21:59:29 UTC 2014
On Wed, 26 Mar 2014, Andrea Venturoli wrote:
>> root@:~ # gpart show /dev/mfid0
>> => 63 1751949249 mfid0 MBR (835G)
>> 63 1751936382 1 freebsd [active] (835G)
>> 1751936445 12867 - free - (6.3M)
>
>> root@:~ # gpart show /dev/mfid0s1
>> => 0 1751936382 mfid0s1 BSD (835G)
>> 0 16777216 4 freebsd-ufs (8.0G)
>> 16777216 33554432 2 freebsd-swap (16G)
>> 50331648 67108864 5 freebsd-ufs (32G)
>> 117440512 33554432 6 freebsd-ufs (16G)
>> 150994944 536870912 7 freebsd-ufs (256G)
>> 687865856 536870912 8 freebsd-ufs (256G)
>> 1224736768 33554432 1 freebsd-ufs (16G)
>> 1258291200 493645182 - free - (235G)
>
> AFAICT the partition are correctly aligned within the slice, but I'm not sure
> about the slice itself.
No. The starting blocks of the slice and partition are added together
to determine the actual start block of the partition:
The first partition begins at 63+0, or 63. 63*512 = 32256, not evenly
divisible by 4K. Or just divide the block number by 8, same thing.
When creating the partitions, use gpart's -a option, and it will add an
offset so the slice is aligned:
gpart add -t freebsd-ufs -a4k -s8g mfid0s1
That will make the first partition start at the first aligned offset in
the slice, probably block 1 or 9.
Use GPT if you can, it simplifies this by not needing the partition with
a partition scheme.
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