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xz: Update the man page about --block-size.
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src/xz/xz.1
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src/xz/xz.1
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@ -5,7 +5,7 @@
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.\" This file has been put into the public domain.
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.\" You can do whatever you want with this file.
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.\"
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.TH XZ 1 "2013-11-12" "Tukaani" "XZ Utils"
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.TH XZ 1 "2014-12-16" "Tukaani" "XZ Utils"
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.
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.SH NAME
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xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
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@ -826,14 +826,39 @@ When compressing to the
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format, split the input data into blocks of
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.I size
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bytes.
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The blocks are compressed independently from each other.
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.\" FIXME: Explain how to these can be used for random access and threading.
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The blocks are compressed independently from each other,
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which helps with multi-threading and
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makes limited random-access decompression possible.
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This option is typically used to override the default
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block size in multi-threaded mode,
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but this option can be used in single-threaded mode too.
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.IP ""
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In multi-threaded mode the sizes of the blocks
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are stored in the block headers.
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This isn't done in single-threaded mode,
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so the encoded output won't be
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identical to that of the multi-threaded mode.
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In multi-threaded mode about three times
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.I size
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bytes will be allocated in each thread for buffering input and output.
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The default
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.I size
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is three times the LZMA2 dictionary size or 1 MiB,
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whichever is more.
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Typically a good value is 2\-4 times
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the size of the LZMA2 dictionary or at least 1 MiB.
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Using
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.I size
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less than the LZMA2 dictionary size is waste of RAM
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because then the LZMA2 dictionary buffer will never get fully used.
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The sizes of the blocks are stored in the block headers,
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which a future version of
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.B xz
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will use for multi-threaded decompression.
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.IP ""
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In single-threaded mode no block splitting is done by default.
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Setting this option doesn't affect memory usage.
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No size information is stored in block headers,
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thus files created in single-threaded mode
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won't be identical to files created in multi-threaded mode.
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The lack of size information also means that a future version of
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.B xz
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won't be able decompress the files in multi-threaded mode.
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.TP
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.BI \-\-block\-list= sizes
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When compressing to the
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