:I'm kind of feeling uneasy that we now "require" a slice. I thought that=
:'s just IBM PC legacy? How about having "vkd0a" =3D=3D "vkd0s0a"? That =
:would look nice again. Or maybe the other way around?
:
:cheers
: simon
We don't require a slice. The internal representations haven't changed,
just the external device names and device numbering. The external
names and numbering had to change because of two problems.
(1) The 'raw' disk also doubled as the labeled disk for the compatibility
slice. e.g. you could 'disklabel da0'. This meant that one
could not access the raw disk directly as a raw disk without going
through some hand waving... there was no way to do it without
the kernel trying to snoop reads and writes for MBR and disklabel
accesses.
(2) The 'raw slice' also doubled as partition 'c' in the slice. e.g.
if you look on a 1.8 or earlier system, 'da0s1' had the same device
number as 'da0s1c'. This meant that we could not distinguish between
a raw slice and partition 'c' within a raw slice.
To fix these problems the compatibility device names and the raw-slice
devices had to be renumbered. That is, in 1.9 now, 'da0' really does
mean a raw disk, the kernel doesn't attempt to process the MBR or
disklabel or snoop reads and writes or enforce reserved areas. 'da0s0'
is the faked compatibility slice (aka for disklabeled disks without
DOS partition tables). They are separate entities now. And, in 1.9,
we distinguish between a raw slice, aka da0s1, and a partition
specification using a disklabel installed on that slice, aka da0s1{a-p}.
Pre 1.9 there was no distinction... you could run disklabel on
anything and the system would silently 'translate' the device to
a device that was label capable, and the system also tried to snoop
MBR and label accesses even for the whole-raw-disk device (e.g. da0),
which created serious confusion. Now we have real device names
representing the labelable raw slices (e.g. da0s1), a faked compatibility
slice (e.g. da0s0) that can be labeled if you don't have an DOS partition
table (otherwise it aliases the first BSD partition just like the
compatibility slices used to, but I might remove that feature), the
partition specifications are properly distinct from the raw slice
name (da0s1 != da0s1c any more), and the disk layer can now avoid
snooping labels or scanning the whole partition table until they
are actually accessed.
-Matt
Matthew Dillon
<dillon@backplane.com>
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