man nbdkit-xz-plugin (1): nbdkit xz plugin

SYNOPSIS

nbdkit xz file=FILENAME.xz

DESCRIPTION

"nbdkit-xz-plugin" is a file serving plugin for nbdkit(1).

It serves the named "FILENAME.xz" over NBD, uncompressing it on the fly. The plugin 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
 =============

xz files are seekable on block boundaries only. Seeking is done by seeking directly to the lower block boundary, then (slowly) uncompressing data until the precise byte is reached.

To get best random access performance, you must prepare your xz files with many small blocks.

To do this, use the --block-size option with a small-ish block size. For example the same image as above 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

This file can be accessed randomly, and at most 16 MB of compressed data will have to be uncompressed to seek to any byte.

Note as you would expect, xz cannot compress quite as efficiently when using a small block size. The space penalty in the above example is < 1% of the compressed file size.

PARAMETERS

file=FILENAME.xz

Serve the file named "FILENAME.xz".

This parameter is required.

maxblock=SIZE

The maximum block size that the plugin will read. The plugin 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.

maxdepth=N

Maximum number of blocks stored in the LRU block cache.

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

The plugin may allocate up to maximum block size in file * maxdepth bytes of memory per connection.

AUTHORS

Richard W.M. Jones

COPYRIGHT

LICENSE

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