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.\" ========================================================================
.\"
.IX Title "nbdkit-pause-filter 1"
.TH nbdkit-pause-filter 1 "2021-01-20" "nbdkit-1.24.1" "NBDKIT"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
nbdkit\-pause\-filter \- pause NBD requests
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit \-\-filter=pause PLUGIN [PLUGIN\-ARGS...] pause\-control=SOCKET
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-pause\-filter\*(C'\fR is a filter for \fBnbdkit\fR\|(1) which can
temporarily stop \s-1NBD\s0 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.
.SS "Control socket"
.IX Subsection "Control socket"
The \f(CW\*(C`pause\-control\*(C'\fR parameter is the name of a Unix domain socket
which the filter listens for commands on.
.PP
To pause \s-1NBD\s0 request processing you send character \f(CW\*(Aqp\*(Aq\fR to the
socket.  Any \s-1NBD\s0 requests received afterwards will hang until you
resume processing.
.PP
To resume processing you send character \f(CW\*(Aqr\*(Aq\fR to the socket.
.PP
So you can know when pausing/resuming has taken effect, the filter
echos back the character over the socket in uppercase (ie. either
\&\f(CW\*(AqP\*(Aq\fR or \f(CW\*(AqR\*(Aq\fR).  When pausing, the \f(CW\*(AqP\*(Aq\fR response is not sent
until all outstanding \s-1NBD\s0 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 \*(L"see\*(R".
.PP
Any unknown commands are ignored.  The filter responds with \f(CW\*(AqX\*(Aq\fR.
.SH "EXAMPLE"
.IX Header "EXAMPLE"
Pick a large file, disk image or \s-1ISO,\s0 serve it over \s-1NBD,\s0 and start
copying it:
.PP
.Vb 3
\& nbdkit \-U \- \-\-filter=pause \-\-filter=rate \e
\&   file BIG_FILE.ISO rate=10M pause\-control=sock \e
\&        \-\-run \*(Aqqemu\-img convert \-p $nbd /var/tmp/out\*(Aq
.Ve
.PP
To cause the copy to appear to hang, do:
.PP
.Vb 1
\& echo p | nc \-U sock
.Ve
.PP
To resume activity:
.PP
.Vb 1
\& echo r | nc \-U sock
.Ve
.SH "PARAMETERS"
.IX Header "PARAMETERS"
.IP "\fBpause\-control=\fR\s-1SOCKET\s0" 4
.IX Item "pause-control=SOCKET"
The Unix domain socket for controlling the filter.
See \*(L"Control socket\*(R" above.
.SH "NOTES"
.IX Header "NOTES"
If you are connecting a kernel client, virtual machine or similar to
nbdkit then only short pauses are tolerated, and you will soon get
timeout errors.  The timeouts are not generated by nbdkit, but by the
client itself.
.PP
The pause filter does not flush requests to disk, although this is a
possible future enhancement.
.PP
A virtual machine with multiple disks connected through multiple
nbdkit instances cannot get a consistent snapshot using this filter,
since even if you send the pause commands to all instances at the same
time they will be processed at slightly different times (and this can
matter if a virtual machine is doing something like \s-1RAID\s0 across the
disks).  This is not something that can be solved at the level of
individual devices, the only way to solve this is at the hypervisor
level.
.SH "FILES"
.IX Header "FILES"
.IP "\fI\f(CI$filterdir\fI/nbdkit\-pause\-filter.so\fR" 4
.IX Item "$filterdir/nbdkit-pause-filter.so"
The filter.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$filterdir\fR.
.SH "VERSION"
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-pause\-filter\*(C'\fR first appeared in nbdkit 1.22.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3),
\&\fBnbdkit\-delay\-filter\fR\|(1),
\&\fBnbdkit\-rate\-filter\fR\|(1),
\&\fBnc\fR\|(1).
.SH "AUTHORS"
.IX Header "AUTHORS"
Richard W.M. Jones
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (C) 2020 Red Hat Inc.
.SH "LICENSE"
.IX Header "LICENSE"
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
.IP "\(bu" 4
Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
.IP "\(bu" 4
Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
.IP "\(bu" 4
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.
.PP
\&\s-1THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS\s0 ''\s-1AS IS\s0'' \s-1AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES\s0 (\s-1INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES\s0; \s-1LOSS OF
USE, DATA, OR PROFITS\s0; \s-1OR BUSINESS INTERRUPTION\s0) \s-1HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT\s0 (\s-1INCLUDING NEGLIGENCE OR OTHERWISE\s0) \s-1ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.\s0