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.\" ========================================================================
.\"
.IX Title "nbdkit-xz-filter 1"
.TH nbdkit-xz-filter 1 "2019-01-26" "nbdkit-1.10.3" "NBDKIT"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
nbdkit\-xz\-filter \- nbdkit xz filter
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit \-\-filter=xz file FILENAME.xz
\&
\& nbdkit \-\-filter=xz curl https://example.com/FILENAME.xz
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-xz\-filter\*(C'\fR is a filter for \fBnbdkit\fR\|(1) which uncompresses
the underlying plugin on the fly.  The filter only supports read-only
connections.
.SS "Getting best random access performance from xz"
.IX Subsection "Getting best random access performance from xz"
\&\fBxz\fR\|(1) files are split into streams and blocks.  Most xz files
contain only one stream which contains one or more blocks.  You can
find out how many streams and blocks are in an xz file by doing:
.PP
.Vb 5
\& $ xz \-\-list winxp.img.xz
\& Strms  Blocks   Compressed Uncompressed  Ratio  Check   Filename
\&     1       1  2,100.0 MiB  6,144.0 MiB  0.342  CRC64   winxp.img.xz
\&    ↑↑↑     ↑↑↑
\&  streams  blocks
.Ve
.PP
xz files are seekable on block boundaries only.  Seeking is done by
seeking directly to the lower block boundary, then uncompressing data
until the precise byte is reached.  As uncompressing xz data is slow,
\&\fBto get best random access performance, you must prepare your xz
files with many small blocks.\fR
.PP
Use the \fI\-\-block\-size\fR option with a small-ish block size.  For
example this is the same image compressed with a 16 \s-1MB\s0 block size:
.PP
.Vb 7
\& $ xz \-\-best \-\-block\-size=16777216 winxp.img
\&             ─────────────────────
\& $ xz \-\-list winxp.img.xz
\& Strms  Blocks   Compressed Uncompressed  Ratio  Check   Filename
\&     1     384  2,120.1 MiB  6,144.0 MiB  0.345  CRC64   winxp.img.xz
\&           ↑↑↑
\&          blocks
.Ve
.PP
This file can be accessed randomly.  At most 16 \s-1MB\s0 will have to be
uncompressed to seek to any byte.
.PP
As you would expect, xz cannot compress as efficiently when using a
smaller block size.  The space penalty in the above example is
< 1% of the compressed file size.
.SH "PARAMETERS"
.IX Header "PARAMETERS"
.IP "\fBxz\-max\-block=\fR\s-1SIZE\s0" 4
.IX Item "xz-max-block=SIZE"
The maximum block size that the filter will read.  The filter will
refuse to read xz files that contain any block larger than this size.
.Sp
See the discussion above about creating xz files with small block
sizes in order to reduce memory usage and increase performance.
.Sp
This parameter is optional.  If not specified it defaults to 512M.
.IP "\fBxz\-max\-depth=\fRN" 4
.IX Item "xz-max-depth=N"
Maximum number of blocks stored in the \s-1LRU\s0 block cache.
.Sp
This parameter is optional.  If not specified it defaults to 8.
.Sp
The filter may allocate up to
maximum block size in file * maxdepth
bytes of memory \fIper connection\fR.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3),
\&\fBnbdkit\-curl\-plugin\fR\|(1),
\&\fBnbdkit\-file\-plugin\fR\|(1),
\&\fBxz\fR\|(1).
.SH "AUTHORS"
.IX Header "AUTHORS"
Richard W.M. Jones
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (C) 2013\-2018 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