NAME

       nbdkit-pause-filter - pause NBD requests

SYNOPSIS

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

DESCRIPTION

       "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.

   Control socket
       The "pause-control" parameter is the name of a Unix domain socket which the filter listens
       for commands on.

       To pause NBD request processing you send character 'p' to the socket.  Any NBD requests
       received afterwards will hang until you resume processing.

       To resume processing you send character 'r' to the socket.

       So you can know when pausing/resuming has taken effect, the filter echos back the
       character over the socket in uppercase (ie. either 'P' or 'R').  When pausing, the 'P'
       response is not sent until all outstanding NBD requests (received before the pause) have
       been completed.  This usually means the plugin is idle, although be aware that it is
       possible for plugins to create background threads and do work that the filter cannot
       "see".

       Any unknown commands are ignored.  The filter responds with 'X'.

EXAMPLE

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

        nbdkit -U - --filter=pause --filter=rate \
          file BIG_FILE.ISO rate=10M pause-control=sock \
               --run 'qemu-img convert -p $nbd /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

PARAMETERS

       pause-control=SOCKET
           The Unix domain socket for controlling the filter.  See "Control socket" above.

NOTES

       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.

FILES

       $filterdir/nbdkit-pause-filter.so
           The filter.

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

VERSION

       "nbdkit-pause-filter" first appeared in nbdkit 1.22.

SEE ALSO

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

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2020 Red Hat Inc.

LICENSE

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