NAME

       nbdkit-xz-filter - nbdkit xz filter

SYNOPSIS

        nbdkit --filter=xz file FILENAME.xz

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

DESCRIPTION

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

   Getting best random access performance from xz
       xz(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, to get best random access performance, you must prepare
       your xz files with many small blocks.

       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.

PARAMETERS

       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
           per connection.

FILES

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

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

VERSION

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

SEE ALSO

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

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2013-2020 Red Hat Inc.

LICENSE

       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.

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