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.IX Title "nbdkit-readahead-filter 1"
.TH nbdkit-readahead-filter 1 2024-01-17 nbdkit-1.36.3 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\-readahead\-filter \- prefetch data ahead of sequential reads
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit \-\-filter=readahead PLUGIN
.Ve
.PP
.Vb 1
\& nbdkit \-\-filter=readahead \-\-filter=cache PLUGIN
.Ve
.PP
.Vb 1
\& nbdkit \-\-filter=readahead \-\-filter=cow PLUGIN cow\-on\-cache=true
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-readahead\-filter\*(C'\fR is a filter that prefetches data when the
client is reading.
.PP
When the client issues a read, this filter issues a parallel prefetch
(\f(CW\*(C`.cache\*(C'\fR) for subsequent data.  Plugins which support this command
will prefetch the data, making sequential reads faster.  For plugins
which do not support this command, you can inject
\&\fBnbdkit\-cache\-filter\fR\|(1) below (after) this filter, giving
approximately the same effect.  \fBnbdkit\-cow\-filter\fR\|(1) can be used
instead of nbdkit-cache-filter, if you add the \f(CW\*(C`cow\-on\-cache=true\*(C'\fR
option.
.PP
The filter uses a simple adaptive algorithm which accelerates
sequential reads and requires no further configuration.
.PP
A similar filter is \fBnbdkit\-scan\-filter\fR\|(1) which reads ahead over
the whole disk, useful if you know that the client will be reading
sequentially across most or all of the disk.
.SS Limitations
.IX Subsection "Limitations"
In a number of significant cases this filter will do nothing.  The
filter will print a warning message if this happens.
.IP "Thread model must be parallel" 4
.IX Item "Thread model must be parallel"
For example \fBnbdkit\-curl\-plugin\fR\|(1) only supports
\&\f(CW\*(C`serialize_requests\*(C'\fR, and so this filter cannot perform prefetches in
parallel with the read requests.
.Sp
We may be able to lift this restriction in future.
.ie n .IP "Underlying filters or plugin must support "".cache"" (prefetch)" 4
.el .IP "Underlying filters or plugin must support \f(CW.cache\fR (prefetch)" 4
.IX Item "Underlying filters or plugin must support .cache (prefetch)"
Very many plugins do not have the concept of prefetching and/or
do not implement the \f(CW\*(C`.cache\*(C'\fR callback, and so there is no
way for this filter to issue prefetches.
.Sp
You can usually get around this by adding \fI\-\-filter=cache\fR after this
filter as explained above.  It may be necessary to limit the total
size of the cache (see "CACHE MAXIMUM SIZE" in \fBnbdkit\-cache\-filter\fR\|(1)).
.IP "Clients and kernels may do readahead already" 4
.IX Item "Clients and kernels may do readahead already"
It may be the case that NBD clients are already issuing
\&\f(CW\*(C`NBD_CMD_CACHE\*(C'\fR (NBD prefetch) commands.  It may also be the case
that your plugin is using local file functions where the kernel is
doing readahead.  In such cases this filter is not necessary and may
be pessimal.
.SH PARAMETERS
.IX Header "PARAMETERS"
There are no parameters specific to nbdkit-readahead-filter.  Any
parameters are passed through to and processed by the underlying
plugin in the normal way.
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$filterdir\fR\fI/nbdkit\-readahead\-filter.so\fR 4
.el .IP \fR\f(CI$filterdir\fR\fI/nbdkit\-readahead\-filter.so\fR 4
.IX Item "$filterdir/nbdkit-readahead-filter.so"
The filter.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$filterdir\fR.
.SH VERSION
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-readahead\-filter\*(C'\fR first appeared in nbdkit 1.12.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-cache\-filter\fR\|(1),
\&\fBnbdkit\-cow\-filter\fR\|(1),
\&\fBnbdkit\-file\-plugin\fR\|(1),
\&\fBnbdkit\-retry\-filter\fR\|(1),
\&\fBnbdkit\-scan\-filter\fR\|(1),
\&\fBnbdkit\-torrent\-plugin\fR\|(1),
\&\fBnbdkit\-vddk\-plugin\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3),
\&\fBqemu\-img\fR\|(1).
.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 ''AS IS'' 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.