On Fri, 21 Jun 2013, Genre Roger wrote:
When I refer to gpart(8) manpage (9.1 release, updated jan 25,2013), I find
the description of partition types a bit confusing.
Considering the case of a MoBo with "old way" Bios (non UEFI), the only
scheme allowed to get a bootable system is MBR. (Note that after booting,
your OS is able to manage gpt-scheme on different providers.)
Sorry, I don't understand. GPT boots fine on a BIOS system. I'm using
it on several systems.
Then, if you speak "bsd-ish", you are able to add on the "MBR"-scheme
provider up to 4 "freebsd-type" slices, each of them containing up to 8
partitions with a BSD filesystem (or swap space).
Yes. This is a partitions-inside-a-slice configuration, used for quite
a while because it provides more partitions than MBR's four, and also
because it allows a standard PC to multi-boot from any of the four MBR
slices. Technically, it's a hack. It's complex and shows that both
partitioning schemes (MBR and bsdlabel) have weaknesses.
Or, if you speak "msdos-ish", you would be able to add on the "MBR" provider
up to 4 "msdos-type" partitions (or 3 + 1 extended ), each of them containing
one filesystem.
The manpage does'nt list explicitly the type for this second case; Oliver try
to use "mbr", decribed as "A partition that is sub-partitioned by a Master
Boot Record (MBR). This type is known as
"!024dee41-33e7-11d3-9d69-0008c781f39f " by GPT.". Such definition is'nt very
explicit.
In general, it's rare to find a FreeBSD system that uses MBR slices
directly because there are only four. But it can be done by combining
one or more of the standard filesystems and not subdividing the slices:
ada0s1: freebsd (/, /var, /tmp, and /usr combined into a single
/ filesystem)
ada0s2: swap
That would leave s3 and s4 available for other uses.
Let me show the example below on my provider ada0 :
$ gpart show
=> 63 976773105 ada0 MBR (465G)
63 136314864 1 freebsd [active] (65G)
136314927 840458241 - free - (400G)
=> 0 136314864 ada0s1 BSD (65G)
0 8388608 1 freebsd-ufs (4.0G)
8388608 16777216 2 freebsd-swap (8.0G)
25165824 16777216 4 freebsd-ufs (8.0G)
41943040 8388608 5 freebsd-ufs (4.0G)
50331648 85983215 6 freebsd-ufs (41G)
136314863 1 - free - (512B)
$ sudo gpart add -t mbr -i 2 -s 8G ada0
Password:
gpart: Invalid argument
MBR is not a type of partition, it is a partitioning method or "scheme"
as the gpart man page calls them. The partitioning scheme is dictated
by the type of system. Standard x86 computers are only going to be able
to use MBR or GPT.
$ sudo gpart add -t \!12 -i 2 -s 8G ada0
ada0s2 added
\!12 is just an MS-DOS partition (FAT32 with LBA).
$ sudo newfs_msdos ada0s2
/dev/ada0s2: 16773056 sectors in 262079 FAT32 clusters (32768
bytes/cluster)
BytesPerSec=512 SecPerClust=64 ResSectors=32 FATs=2 Media=0xf0
SecPerTrack=63 Heads=16 HiddenSecs=0 HugeSectors=16777215 FATsecs=2048
RootCluster=2 FSInfo=1 Backup=2
$ sudo gpart add -t freebsd -i 3 -s 8G ada0
ada0s3 added
$ gpart show ada0
=> 63 976773105 ada0 MBR (465G)
63 136314864 1 freebsd [active] (65G)
136314927 16777215 2 !12 (8G)
153092142 16777215 3 freebsd (8G)
169869357 806903811 - free - (384G)
$ sudo newfs_msdos ada0s3
/dev/ada0s3: 16773056 sectors in 262079 FAT32 clusters (32768
bytes/cluster)
BytesPerSec=512 SecPerClust=64 ResSectors=32 FATs=2 Media=0xf0
SecPerTrack=63 Heads=16 HiddenSecs=0 HugeSectors=16777215 FATsecs=2048
RootCluster=2 FSInfo=1 Backup=2
$
this example show that, if you want a "msdos" or "fat32" partition, both the
type "\!12" or "freebsd" allows you to create a partition where you could
install a msdos filesystem (but the "mbr" does not).
It shows that MBR is a partitioning scheme, not a partition type. It
also demonstrates that newfs_msdos does not check the partition type of
the partition on which it is creating a filesystem. Neither does newfs.
That is appropriate behavior. The partition type is so that bootcode
and other software that deals with partitions does not have to guess
what is on a particular partition, but has it identified explicitly.
Such behaviour is not surprising, (using freebsd-type to format as msdos
means you don't use the space allowed for labeling);
I don't understand what you are saying there. Offhand, I can't think of
any reason to misidentify a partition intentionally.
but what is really the purpose of the listed "mbr"-type in the gpart(8) manpage?
MBR is a partitioning scheme, comparable to GPT but more limited. APM
is a partitioning scheme for PPC Macs, VTOC8 is a partitioning scheme
for Sparc.
The partitioning scheme determines which types of partitions are
allowed. For MBR, there are lots of partition types for variations for
MS-DOS (including type 12), one (really) for NTFS, one for FreeBSD:
http://en.wikipedia.org/wiki/Partition_type
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