nbdkit-xz-filter(1)

📦
version 1.48.1 · 14 versions
, in fedora 44

💖 nbdkit-xz-filter — bestie's guide to the xz filter 💖

okay so welcome to nbdkit-xz-filter, a filter for the nbdkit manual page, and yeah it's part of nbdkit (the man page lives under nbdkit-xz-filter(1) if you're being formal, which... we are not). ✨

🌸 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) and the way it works is that it uncompresses the underlying plugin on the fly, like a little fairy doing the work in real time 🧚. but hold up — the filter only supports read-only connections. no writing, no editing, no chaos. it's giving museum, not workshop. 💅

✨ Getting best random access performance from xz ✨

so here's the tea. xz(1) files are split into streams and blocks. Most xz files contain only one stream, and that one stream contains one or more blocks. lowkey iconic structure.

you can find out how many streams and blocks are in an xz file by doing this:

$ 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

see those arrows? that's the vibe check. 👀

xz files are seekable on block boundaries only. so how does seeking even work? it seeks directly to the lower block boundary, then uncompresses data until the precise byte is reached. and because uncompressing xz data is slow (no shade, it's just doing a lot), 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

and now? this file can be accessed randomly, bestie!! 🎉 At most 16 MB will have to be uncompressed to seek to any byte. slay.

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. so it's a tiny little tradeoff for a huge little glow-up. 💖

🎀 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. (like, hard no. 🙅)

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. (so like, do the math before you go feral with the numbers, okay? 😭)

🍓 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. so it's got lore. it's got history. it's got main character energy. 🌈

🌸 SEE ALSO 🌸

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).

go read the whole friend group, they're all iconic. 🫶

💜 AUTHORS 💜

Richard W.M. Jones

Copyright Red Hat

📜 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.
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.
fr fr, read the license. it's not a vibe, it's a requirement. 💀

and that's the whole page!! this one's from nbdkit-1.48.1, dated 2026-08-29 — mwah, no notes, go uncompress something 💖✨

📦 where this page comes from