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[Qemu-devel] Few Qs related to using qemu-nbd for exporting snapshot

From: Deepak C Shetty
Subject: [Qemu-devel] Few Qs related to using qemu-nbd for exporting snapshot
Date: Wed, 08 May 2013 10:30:29 +0530
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120911 Thunderbird/15.0.1

Hi Anthony & the List,

    I am looking at using qemu-nbd to export an existing snapshot to a Backup 
virtual appliance (VA) and had the following Qs in that context...

1) Exporting an image using unix socket (-k option) Vs using
--connect=/dev/nbd0, which one is better / preferred ? The idea being
once exported, the disk will be hotplugged into Backup VA which will
then use it as a src for backup operation.

2) If i use the --connect option, it requires the nbd kernel module to
be loaded and i also get a nice --disconnect way of terminating the nbd

Using -k option qemu-nbd doesn't need the nbd kernel module, but the
cmdline blocks and sending SIGTERM is the only way to terminate the nbd
export... Is there a better way to terminate qemu-nbd if using the -k
option ? If not, i assume SIGTERM would gracefully shut down qemu-nbd ?

3) I was not able to get qemu-nbd running without the -t (persistent)
option. Ideally without -t would be suited, since there is only one
client/connection per nbd export and the nbd server would nicely shut
itself down.. but i get the below error ....

server as...
$ qemu-nbd -k /tmp/nbds1 ./debian_lenny_i386_standard.qcow2

client as...
$ qemu-system-x86_64 -m 1024 -drive file=nbd:unix:/tmp/nbds1
connect(unix:/tmp/nbds1): No such file or directory
qemu-system-x86_64: -drive file=nbd:unix:/tmp/nbds1: could not open disk
image nbd:unix:/tmp/nbds1: No such file or directory

nbds1 is indeed there...
srwxrwxr-x. 1 dpkshetty dpkshetty 0 May  3 14:24 /tmp/nbds1

and then the server returns to the shell.. echo $? is 0


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