Jerry Vonau wrote:
Yeah, you could easily just do a snapshot and then set LIVE_BLOCK to
that.
Got a magic string? ;-) DMC, Can you point me in the direction please?
I'll play around...
Here is an example. Note, that trying to do the below on your normal
rootfs would not be doable (though maybe with snapshot-origin), but it
would be doable with the livecd because you are playing with devices
already under devicemapper's control. Also, here is an absolute gem of
documentation, which took me a very, very long time to truly understand
all of.
http://linuxgazette.net/114/kapil.html
# this is the device you want to be able to write
# to without actually writing to.
base_device=/dev/sdZ
# this is the name of the new device that will be
# created which can be used as a writable version
# of the base device, without actually writing to it.
# It will show up as /dev/mapper/${cow_device}
cow_device=mycow
# /dev/shm is a tmpfs available by default.
# a file on a writable disk would work as well
overlay_file=/dev/shm/overlay
# this sets up an overlay file that is a 1G sparse
# file. I.e. an ls -l will show, 1G, but a du -chs
# will show much of it has actually been written to
dd if=/dev/zero of=${overlay_file} bs=1k \
count=1 seek=$(( 1024 * 1024 ))
# set up a loop/block device for the overlay file
overlay_loop_device=$( losetup -f )
losetup ${overlay_loop_device} ${overlay_file}
# this is just extra paranoia
# set the base device to read only
blockdev --setro ${base_device}
# now, create the copy-on-write device
# (wrapped for email, not sure if this wrapping
# will actually work)
dmsetup create ${cow_device} --table \
"0 $( blockdev --getsize ${base_device} ) \
snapshot ${base_device} ${overlay_loop_device} p 8"
# inspect stuff
dmsetup status ${cow_device}
ls -lh ${overlay_file}
du -hs ${overlay_file}
# now, do stuff
dd if=/dev/zero of=/dev/mapper/${cow_device} bs=1M count=10
# now inspect stuff for fun
dmsetup status ${cow_device}
ls -lh ${overlay_file}
du -hs ${overlay_file}
# now, tear everything down, no harm no foul.
dmsetup remove ${cow_device}
blockdev --setrw ${base_device}
losetup -d ${overlay_loop_device}
rm -f ${overlay_file}
-dmc
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