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.IX Title "nbdkit-delay-filter 1"
.TH nbdkit-delay-filter 1 2026-04-03 nbdkit-1.47.7 NBDKIT
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH NAME
nbdkit\-delay\-filter \- nbdkit delay filter
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 11
\& nbdkit \-\-filter=delay plugin [plugin\-args ...]
\&          delay\-read=DELAY
\&          delay\-write=DELAY
\&          delay\-zero=DELAY
\&          delay\-trim=DELAY
\&          delay\-extents=DELAY
\&          delay\-cache=DELAY
\&          delay\-open=DELAY
\&          delay\-close=DELAY
\&          delay\-fast\-zero=BOOL
\&          delay\-trigger=FILENAME
.Ve
.PP
.Vb 1
\& nbdkit \-\-filter=delay plugin rdelay=DELAY wdelay=DELAY [plugin\-args...]
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-delay\-filter\*(C'\fR is a filter for \fBnbdkit\fR\|(1) that delays read
and write requests.  This is used to simulate a slow or remote server,
or to test certain kinds of race conditions in Linux.  To limit server
bandwidth use \fBnbdkit\-rate\-filter\fR\|(1) instead.  To simulate a
spinning disk see \fBnbdkit\-spinning\-filter\fR\|(1).
.SH EXAMPLES
.IX Header "EXAMPLES"
Delays reads and writes by 100ms:
.PP
.Vb 1
\& nbdkit \-\-filter=delay file disk.img rdelay=100ms wdelay=100ms
.Ve
.PP
Delay only zero operations by 1 second, nothing else is affected:
.PP
.Vb 1
\& nbdkit \-\-filter=delay file disk.img delay\-zero=1
.Ve
.SH "DELAY FORMAT"
.IX Header "DELAY FORMAT"
In nbdkit ≥ 1.40, \fBnbdkit_parse_delay\fR\|(3) is used to parse the
delay parameters, so a wide variety of formats are accepted, such as:
.IP delay\-read=1.5 4
.IX Item "delay-read=1.5"
.PD 0
.IP delay\-write=100ms 4
.IX Item "delay-write=100ms"
.IP delay\-zero=1.2μs 4
.IX Item "delay-zero=1.2μs"
.IP delay\-trim=1000ns 4
.IX Item "delay-trim=1000ns"
.PD
.PP
In nbdkit ≤ 1.38, only a whole number of seconds, or a whole
number of milliseconds (if suffixed by \f(CW\*(C`ms\*(C'\fR) were allowed.
.SH "TRIGGERED DELAYS"
.IX Header "TRIGGERED DELAYS"
Using the \f(CW\*(C`delay\-trigger=FILENAME\*(C'\fR option, you can disable this
filter until the named \fIFILENAME\fR is created.  If the file is
deleted, then the filter is disabled again.  This allows you to boot
up a virtual machine before turning on delays.
.SH PARAMETERS
.IX Header "PARAMETERS"
.IP \fBdelay\-cache=\fRDELAY 4
.IX Item "delay-cache=DELAY"
(nbdkit ≥ 1.14)
.Sp
Delay advisory cache operations by \f(CW\*(C`DELAY\*(C'\fR.
.IP \fBdelay\-close=\fRDELAY 4
.IX Item "delay-close=DELAY"
(nbdkit ≥ 1.28)
.Sp
Delay close (client disconnection) by \f(CW\*(C`DELAY\*(C'\fR.  This can also cause
server shutdown to be delayed if clients are connected at the time.
This only affects clients that gracefully disconnect (using
\&\f(CW\*(C`NBD_CMD_DISC\*(C'\fR / libnbd function \fBnbd_shutdown\fR\|(3)).  Clients that
abruptly disconnect from the server cannot be delayed.
.IP \fBdelay\-extents=\fRDELAY 4
.IX Item "delay-extents=DELAY"
(nbdkit ≥ 1.12)
.Sp
Delay block status (extents) operations by \f(CW\*(C`DELAY\*(C'\fR.
.IP \fBdelay\-fast\-zero=\fRBOOL 4
.IX Item "delay-fast-zero=BOOL"
(nbdkit ≥ 1.16)
.Sp
The NBD specification documents an extension called fast zero, in
which the client may request that a server should reply with
\&\f(CW\*(C`ENOTSUP\*(C'\fR as soon as possible if the zero operation offers no real
speedup over a corresponding write.  By default, this parameter is
true, and fast zero requests are serviced by the plugin after the same
delay as any other zero request; but setting this parameter to false
instantly fails a fast zero response without waiting for or consulting
the plugin.
.IP \fBdelay\-open=\fRDELAY 4
.IX Item "delay-open=DELAY"
(nbdkit ≥ 1.28)
.Sp
Delay open (client connection) by \f(CW\*(C`DELAY\*(C'\fR.
.IP \fBdelay\-read=\fRDELAY 4
.IX Item "delay-read=DELAY"
.PD 0
.IP \fBrdelay=\fRDELAY 4
.IX Item "rdelay=DELAY"
.PD
Delay read operations by \f(CW\*(C`DELAY\*(C'\fR.
.Sp
The two forms \f(CW\*(C`delay\-read\*(C'\fR and \f(CW\*(C`rdelay\*(C'\fR work identically.
.IP \fBdelay\-trigger=\fRFILENAME 4
.IX Item "delay-trigger=FILENAME"
(nbdkit ≥ 1.46)
.Sp
If this option is used, the filter is only enabled when the file
called \fIFILENAME\fR exists.  Therefore this file may be created and
deleted while nbdkit is running to enable and disable delays.
.IP \fBdelay\-trim=\fRDELAY 4
.IX Item "delay-trim=DELAY"
(nbdkit ≥ 1.10)
.Sp
Delay trim/discard operations by \f(CW\*(C`DELAY\*(C'\fR.
.IP \fBdelay\-write=\fRDELAY 4
.IX Item "delay-write=DELAY"
Delay write operations by \f(CW\*(C`DELAY\*(C'\fR.
.IP \fBdelay\-zero=\fRDELAY 4
.IX Item "delay-zero=DELAY"
(nbdkit ≥ 1.10)
.Sp
Delay zero operations by \f(CW\*(C`DELAY\*(C'\fR.  See also the \f(CW\*(C`delay\-fast\-zero\*(C'\fR
option.
.IP \fBwdelay=\fRDELAY 4
.IX Item "wdelay=DELAY"
Delay write, zero and trim operations by \f(CW\*(C`DELAY\*(C'\fR.  (This is the same
as setting \f(CW\*(C`delay\-write\*(C'\fR, \f(CW\*(C`delay\-zero\*(C'\fR and \f(CW\*(C`delay\-trim\*(C'\fR options
together.)
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$filterdir\fR\fI/nbdkit\-delay\-filter.so\fR 4
.el .IP \fR\f(CI$filterdir\fR\fI/nbdkit\-delay\-filter.so\fR 4
.IX Item "$filterdir/nbdkit-delay-filter.so"
The filter.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$filterdir\fR.
.SH VERSION
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-delay\-filter\*(C'\fR first appeared in nbdkit 1.2, based on earlier
\&\f(CW\*(C`rdelay\*(C'\fR and \f(CW\*(C`wdelay\*(C'\fR options in \fBnbdkit\-file\-plugin\fR\|(1).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3),
\&\fBnbdkit\-pause\-filter\fR\|(1),
\&\fBnbdkit\-rate\-filter\fR\|(1),
\&\fBnbdkit\-spinning\-filter\fR\|(1),
\&\fBnbdkit_parse_delay\fR\|(3).
.SH AUTHORS
.IX Header "AUTHORS"
Richard W.M. Jones
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.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
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS \*(Aq\*(AqAS IS\*(Aq\*(Aq AND
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 (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
