nbdkit-pause-filter - Man Page

 nbdkit --filter=pause PLUGIN [PLUGIN-ARGS...] pause-control=SOCKET

nbdkit-pause-filter is a filter for nbdkit(1) which can temporarily stop NBD requests from being handled by nbdkit, and later resume them.  This filter can be used if you need to take a snapshot of the underlying storage.

Pick a large file, disk image or ISO, serve it over NBD, and start copying it:

 nbdkit --filter=pause --filter=rate \
   file BIG_FILE.ISO rate=10M pause-control=sock \
        --run 'qemu-img convert -p "$uri" /var/tmp/out'

To cause the copy to appear to hang, do:

 echo p | nc -U sock

To resume activity:

 echo r | nc -U sock

pause-control=SOCKET

The Unix domain socket for controlling the filter. See "Control socket" above.

If you are connecting a kernel client, virtual machine or similar to nbdkit then only short pauses are tolerated, and you will soon get timeout errors.  The timeouts are not generated by nbdkit, but by the client itself.

The pause filter does not flush requests to disk, although this is a possible future enhancement.

A virtual machine with multiple disks connected through multiple nbdkit instances cannot get a consistent snapshot using this filter, since even if you send the pause commands to all instances at the same time they will be processed at slightly different times (and this can matter if a virtual machine is doing something like RAID across the disks).  This is not something that can be solved at the level of individual devices, the only way to solve this is at the hypervisor level.

$filterdir/nbdkit-pause-filter.so

The filter.

Use nbdkit --dump-config to find the location of $filterdir.

nbdkit-pause-filter first appeared in nbdkit 1.22.

nbdkit(1), nbdkit-filter(3), nbdkit-delay-filter(1), nbdkit-rate-filter(1), nc(1).

Richard W.M. Jones

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nbdkit(1), nbdkit-delay-filter(1), nbdkit-filter(3), nbdkit-rate-filter(1), nbdkit-release-notes-1.22(1).