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support for writing file systems on sparse disk images
Yesterday I wanted to write to a file system which was on a sparse
disk image. Normally I would have just used a rump mount, but since the
file system was not at offset = 0 in the file, this failed. vnd wasn't
helpful either: it allowed me to mount the image and then I got weird
errors when writing.
I just finished adding disklabel support to the various rump_fs utilities.
Due to them using the option parsing code of the real mount_fs utilities,
I decided to signal the label number with a magic parameter at the end
of the device (yes, this really simplifies things currently).
Let's assume you're looking at wd0.img. Here's how it works:
=== SNIP ===
golem> disklabel wd0.img
# size offset fstype [fsize bsize cpg/sgs]
a: 719712 63 4.2BSD 1024 8192 0 # (Cyl. 0*- 714*)
b: 66657 719775 swap # (Cyl. 714*- 780*)
c: 786369 63 unused 0 0 # (Cyl. 0*- 780*)
d: 786432 0 unused 0 0 # (Cyl. 0 - 780*)
golem> rump_ffs wd0.img%PART:a% /puffs
rump_ffs: "wd0.img" is a non-resolved or relative path.
rump_ffs: using "/usr/home/pooka/wd0.img" instead.
golem> df /puffs
Filesystem 1K-blocks Used Avail %Cap Mounted on
/usr/home/pooka/wd0.img%PART:a% 338471 256211 65337 79% /puffs
=== SNIP ===
So you specify the partition number as %PART:n%, where is n is obviously
the partition number. Other than that, things work as usual. No vnconfig
p.s. once the generic fs-mounting SoC code from this summer is merged,
this can be handled much cleaner, e.g. with -o partition=n or something
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