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compress: docs

This commit is contained in:
Marian Beermann 2017-03-31 13:39:54 +02:00
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commit 88647595ac
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src/borg/compress.pyx
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"""
borg.compress
=============
Compression is applied to chunks after ID hashing (so the ID is a direct function of the
plain chunk, compression is irrelevant to it), and of course before encryption.
Borg has a flexible scheme for deciding which compression to use for chunks.
First, there is a global default set by the --compression command line option,
which sets the .compressor attribute on the Key.
For chunks that emanate from files CompressionDecider1 may set a specific
Compressor based on patterns (this is the --compression-from option). This is stored
as a Compressor instance in the "compress" key in the Chunk's meta dictionary.
When compressing either the Compressor specified in the Chunk's meta dictionary
is used, or the default Compressor of the key.
The "auto" mode (e.g. --compression auto,lzma,4) is implemented as a meta Compressor,
meaning that Auto acts like a Compressor, but defers actual work to others (namely
LZ4 as a heuristic whether compression is worth it, and the specified Compressor
for the actual compression).
"""
import zlib
from collections import namedtuple