plucky (1) nbdkit-pause-filter.1.gz

Provided by: nbdkit_1.42.0-1ubuntu1_amd64 bug

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 --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


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 Red Hat


LICENSE
       Redistribution and use in source and binary forms, with or without modification, are permitted provided
       that the following conditions are met:

       •   Redistributions of source code must retain the above copyright notice, this list of conditions and
           the following disclaimer.

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           the following disclaimer in the documentation and/or other materials provided with the distribution.

       •   Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote
           products derived from this software without specific prior written permission.

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