nbdkit-xz-filter - nbdkit xz filter

       Richard W.M. Jones

       Copyright Red Hat

       "nbdkit-xz-filter" is a filter for nbdkit(1) which uncompresses the underlying plugin on the fly.  The
       filter only supports read-only connections.

   Gettingbestrandomaccessperformancefromxzxz(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:

        $ 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

       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,
       togetbestrandomaccessperformance,youmustprepareyourxzfileswithmanysmallblocks.

       Use the --block-size option with a small-ish block size.  For example this is the same image compressed
       with a 16 MB block size:

        $ 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

       This file can be accessed randomly.  At most 16 MB will have to be uncompressed to seek to any byte.

       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.

$filterdir/nbdkit-xz-filter.so
           The filter.

           Use "nbdkit --dump-config" to find the location of $filterdir.

       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.

       •   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.

       •   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.

       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.

nbdkit-1.42.2                                      2025-04-02                                nbdkit-xz-filter(1)

       nbdkit-xz-filter - nbdkit xz filter

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.

           See the discussion above about creating xz files with small block sizes in  order  to  reduce  memory
           usage and increase performance.

           This parameter is optional.  If not specified it defaults to 512M.

       xz-max-depth=N
           Maximum number of blocks stored in the LRU block cache.

           This parameter is optional.  If not specified it defaults to 8.

           The filter may allocate up to maximum block size in file × maxdepth bytes of memory perconnection.

nbdkit(1),   nbdkit-filter(3),   nbdkit-curl-plugin(1),   nbdkit-file-plugin(1),   nbdkit-gzip-filter(1),
       nbdkit-lzip-filter(1), nbdkit-bzip2-filter(1), xz(1).

        nbdkit --filter=xz file FILENAME.xz

        nbdkit --filter=xz curl https://example.com/FILENAME.xz

       "nbdkit-xz-filter"  first  appeared  in  nbdkit  1.10.   It  is derived from an earlier plugin that first
       appeared in nbdkit 1.0.