Re: Mounting filesystems with blktap2

[John Weekes <lists.xen@nuclearfallout.net>]

On 2/17/2012 9:54 PM, Matthew Hook wrote:
> For those wanting to know how to mount filesystems through blktap2
> here is how you do it, as I couldn't find it fully documented.
> The instructions on the blktap2 readme file are rather out of date.
>
>
> You can mount them under Dom0 like so.
>
> e.g. to mount a vhd file,
> sudo xm block-attach 0 tap2:vhd:/home/xen/xenwin7-persist.vhd  xvdb w 0
>
> Now the device should appear under /dev
>
> ls /dev/xvdb*
> /dev/xvdb  /dev/xvdb1  /dev/xvdb2
>
> sudo mount /dev/xvdb2 /mnt
>
> Mounts the first partition on xvdb to the path /mnt

You can also mount with tap-ctl and kpartx, but it's a little more 
complicated:

# tap-ctl allocate
/dev/xen/blktap-2/tapdev7
# tap-ctl spawn
tapdisk spawned with pid 28746
# tap-ctl attach -p 28746 -m 7
# tap-ctl open -p 28746 -m 7 -a aio:/somewhere/some_example_image.img
# kpartx -a /dev/xen/blktap-2/tapdev7
# ls /dev/mapper/tapdev7*
/dev/mapper/tapdev7p1  /dev/mapper/tapdev7p2
# mount /dev/mapper/tapdev7p1 /mnt
# umount /mnt
# kpartx -d /dev/mapper/tapdev7p1
# tap-ctl close -p 28746 -m 7
# tap-ctl detach -p 28746 -m 7
# tap-ctl free -m 7

(Is this a better way? I don't actually know.)

Note that when using the tap-ctl method in scripts, I've found that it's 
important to intersperse "sync" commands and slight timeouts before 
umount/kpart -d/tap-ctl close, as otherwise you will sometimes end up 
with the tapdisk2 process hung in an uninterruptible disk sleep and 
unable to be queried (causing havoc when it comes to trying to trying to 
view the process list). At least, under 4.1.

-John