NAME¶
nbdkit-xz-plugin - 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.
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.
SEE ALSO¶
nbdkit(1),
nbdkit-plugin(3),
xz(1).
AUTHORS¶
Richard W.M. Jones
COPYRIGHT¶
Copyright (C) 2013 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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