488 lines
25 KiB
Markdown
488 lines
25 KiB
Markdown
Pcompress
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=========
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Copyright (C) 2012-2013 Moinak Ghosh. All rights reserved.
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Use is subject to license terms.
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moinakg (_at) gma1l _dot com.
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Comments, suggestions, code, rants etc are welcome.
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Pcompress is an archiver that also does compression and decompression in
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parallel by splitting input data into chunks. It has a modular structure
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and includes support for multiple algorithms like LZMA, Bzip2, PPMD, etc,
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with SKEIN/SHA checksums for data integrity. Compression algorithms are
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selected based on the file type to maximize compression gains using a file
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and data anaylis based adaptive technique. It also includes various data
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transformation filters to improve compression.
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It also implements Variable Block Deduplication and Delta Compression
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features based on a Polynomial Fingerprinting scheme. Delta Compression
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is done via the widely popular bsdiff algorithm. Similarity is detected
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using a technique based on MinHashing. Deduplication metadata is also
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compressed to reduce overheads. In addition to all these it can internally
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split chunks at file and rabin boundaries to help Dedupe and compression.
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It has low metadata overhead and overlaps I/O and compression to achieve
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maximum parallelism. It also bundles a simple slab allocator to speed
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repeated allocation of similar chunks. It can work in pipe mode, reading
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from stdin and writing to stdout. SIMD vector optimizations using the x86
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SSE instruction set are used to speed up various operations. Finally it
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supports 14 compression levels to allow for ultra compression parameters
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in some algorithms.
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Pcompress also supports encryption via AES, Salsa20 and uses Scrypt from
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Tarsnap for Password Based Key generation. A unique key is generated per
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session even if the same password is used and HMAC is used to do authentication.
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Links of Interest
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=================
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Project Home Page: http://moinakg.github.io/pcompress/
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http://moinakg.github.io/pcompress/#deduplication-chunking-analysis
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http://moinakg.github.io/pcompress/#compression-benchmarks
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http://moinakg.wordpress.com/2013/04/26/pcompress-2-0-with-global-deduplication/
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http://moinakg.wordpress.com/2013/03/26/coordinated-parallelism-using-semaphores/
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http://moinakg.wordpress.com/2013/06/11/architecture-for-a-deduplicated-archival-store-part-1/
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http://moinakg.wordpress.com/2013/06/15/architecture-for-a-deduplicated-archival-store-part-2/
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Standard Usage
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==============
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Standard usage consists of a few common options to control basic behavior. A variety of
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parameters including global deduplication are automatically set based on the compression
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level.
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Archiving
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---------
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pcompress -a [-v] [-l <compress level>] [-s <chunk size>] [-c <algorithm>]
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[<file1> <directory1> <file2> ...] [-t <number>] [-S <chunk checksum>]
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<archive filename or '-'>
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Archives a given set of files and/or directories into a compressed PAX archive. The
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PAX datastream is encoded into a custom format compressed file that can only be
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handled by Pcompress.
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-a Enables archive mode where pathnames specified in the command line are
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archived using LibArchive and then compressed.
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-l <compress level>
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Select a compression level from 1 (least compression, fastest) to 14
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(ultra compression, slow). Default: 6
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-s <chunk size>
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Archive data is split into chunks that are processed in parallel. This value
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specifies the maximum chunk size. Blocks may be smaller than this. Values
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can be in bytes or <number><suffix> format where suffix can be k - KB, m - MB,
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g - GB. Default: 8m
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Larger chunks can produce better compression at the cost of memory.
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-c <algorithm>
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Specifies the compression algorithm to use. Default algorithm when archiving
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is adapt2 (Second Adaptive Mode). This is the ideal mode for archiving giving
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best compression gains. However adapt (Adaptive Mode) can be used which is a
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little faster but give lower compression gains.
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Other algorithms can be used if all the files are of the same known type. For
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example ppmd (slow) or libbsc (fast) can be used if all the files only have
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ASCII text. See section "Compression Algorithms" for details.
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-v Enables verbose mode where each file/directory is printed as it is processed.
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-t <number>
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Sets the number of threads that Pcompress can use. Pcompress automatically
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uses thread count = core count. However with larger chunk size (-s option)
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and/or ultra compression levels, large amounts of memory can be used. In this
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case thread count can be reduced to reduce memory consumption.
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-S <chunk checksum>
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Specify then chunk checksum to use. Default: BLAKE256. The following checksums
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are available:
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CRC64 - Extremely Fast 64-bit CRC from LZMA SDK.
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SHA256 - SHA512/256 version of Intel's optimized (SSE,AVX) SHA2 for x86.
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SHA512 - SHA512 version of Intel's optimized (SSE,AVX) SHA2 for x86.
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KECCAK256 - Official 256-bit NIST SHA3 optimized implementation.
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KECCAK512 - Official 512-bit NIST SHA3 optimized implementation.
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BLAKE256 - Very fast 256-bit BLAKE2, derived from the NIST SHA3
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runner-up BLAKE.
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BLAKE512 - Very fast 256-bit BLAKE2, derived from the NIST SHA3
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runner-up BLAKE.
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The fastest checksum is the BLAKE2 family.
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<archive filename>
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Pathname of the resulting archive. A '.pz' extension is automatically added
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if not already present. This can also be specified as '-' in order to send
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the compressed archive stream to stdout.
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Single File Compression
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-----------------------
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pcompress -c <algorithm> [-l <compress level>] [-s <chunk size>] [-p] [<file>]
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[-t <number>] [-S <chunk checksum>] [<target file or '-'>]
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Takes a single file as input and produces a compressed file. Archiving is not performed.
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This can also work in streaming mode.
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-c <algorithm>
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See above. Also see section "Compression Algorithms" for details.
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-l <compress level>
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-s <chunk size>
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-t <number>
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-S <chunk checksum>
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See above.
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Note: In singe file compression mode with adapt2 or adapt algorithm, larger
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chunks may not produce better compression. Smaller chunks can result
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in better data analysis here.
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-p Make Pcompress work in streaming mode. Data is ingested via stdin
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compressed and output via stdout. No filenames are used.
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<target file>
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Pathname of the compressed file to be created. This can be '-' to send the
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compressed data to stdout.
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Decompression and Archive extraction
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------------------------------------
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pcompress -d <compressed file or '-'> [-m] [-K] [-i] [<target file or directory>]
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-m Enable restoring *all* permissions, ACLs, Extended Attributes etc.
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Equivalent to the '-p' option in tar. Ownership is only extracted if run as
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root user.
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-K Do not overwrite newer files.
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-i Only list contents of the archive, do not extract.
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-m and -K are only meaningful if the compressed file is an archive. For single file
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compressed mode these options are ignored.
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<compressed file>
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Specifies the compressed file or archive. This can be '-' to indicate reading
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from stdin while write goes to <target file>
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<target file or directory>
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This can be a filename or a directory depending on how the archive was created.
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If single file compression was used then this can be the name of the target
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file that will hold the uncompressed data.
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If this is omitted then an output file is created by appending '.out' to the
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compressed filename.
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If Archiving was done then this should be the name of a directory into which
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extracted files are restored. The directory is created if it does not exist.
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If this is omitted the files are extracted into the current directory.
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Compression Algorithms
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======================
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lzfx - Fast, average compression. At high compression levels this can be faster
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than LZ4.
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Effective Levels: 1 - 5
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lz4 - Very Fast, sometimes better compression than LZFX.
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Effective Levels: 1 - 3
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zlib - Fast, better compression.
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Effective Levels: 1 - 9
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bzip2 - Slow, much better compression than Zlib.
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Effective Levels: 1 - 9
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lzma - Very slow. Extreme compression. Recommended: Use lzmaMt variant mentioned
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below.
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Effective Levels: 1 - 14
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Till level 9 it is standard LZMA parameters. Levels 10 - 12 use
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more memory and higher match iterations so are slower. Levels
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13 and 14 use larger dictionaries upto 256MB and really suck up
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RAM. Use these levels only if you have at the minimum 4GB RAM on
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your system.
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lzmaMt - This is the multithreaded variant of lzma and typically runs faster.
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However in a few cases this can produce slightly lesser compression
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gain.
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PPMD - Slow. Extreme compression for Text, average compression for binary.
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In addition PPMD decompression time is also high for large chunks.
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This requires lots of RAM similar to LZMA. PPMd requires
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at least 64MB X core-count more memory than the other modes.
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Effective Levels: 1 - 14.
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Adapt - Synthetic mode with text/binary detection. For pure text data PPMD is
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used otherwise Bzip2 is selected per chunk.
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Effective Levels: 1 - 14
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Adapt2 - Slower synthetic mode. For pure text data PPMD is otherwise LZMA is
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applied. Can give very good compression ratio when splitting file
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into multiple chunks.
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Effective Levels: 1 - 14
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Since both LZMA and PPMD are used together memory requirements are
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large especially if you are also using extreme levels above 10. For
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example with 100MB chunks, Level 14, 2 threads and with or without
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dedupe, it uses upto 2.5GB physical RAM (RSS).
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none - No compression. This is only meaningful with -G or -D. So Dedupe
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can be done for post-processing with an external utility.
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Enabled features based on Compression Level
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===========================================
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1 to 3 - No features, just compression and archiving, if needed.
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4 - Global Deduplication with avg block size of 8KB.
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5 - Global Dedup block size 8KB, Adaptive Delta Encoding.
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6 to 8 - Global Dedup block size reduced to 4KB, Adaptive Delta Encoding.
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9 - Global Dedup block size reduced to 2KB, Adaptive Delta Encoding, Dispack.
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10 - Global Dedup block size 2KB, Adaptive Delta Encoding with extra rounds, Dispack,
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LZP Preprocessing
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10 - 14 - Global Dedup block size 2KB, Adaptive Delta Encoding with extra rounds, Dispack,
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LZP Preprocessing, PackJPG filter for Jpegs.
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Encryption
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==========
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Pcompress supports encryption and authentication in both archive and single-file
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compresion modes. Encryption options are discussed below.
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NOTE: When using pipe-mode via -p the only way to provide a password is to use '-w'.
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See below.
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-e <ALGO>
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Encrypt chunks using the given encryption algorithm. The algo parameter
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can be one of AES or SALSA20. Both are used in CTR stream encryption
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mode.
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The password can be prompted from the user or read from a file. Unique
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keys are generated every time pcompress is run even when giving the same
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password. Of course enough info is stored in the compresse file so that
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the key used for the file can be re-created given the correct password.
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Default key length if 256 bits but can be reduced to 128 bits using the
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'-k' option.
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The Scrypt algorithm from Tarsnap is used
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(See: http://www.tarsnap.com/scrypt.html) for generating keys from
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passwords. The CTR mode AES mechanism from Tarsnap is also utilized.
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-w <pathname>
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Provide a file which contains the encryption password. This file must
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be readable and writable since it is zeroed out after the password is
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read.
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-k <key length>
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Specify the key length. Can be 16 for 128 bit keys or 32 for 256 bit
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keys. Default value is 32 for 256 bit keys.
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Advanced usage
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==============
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A variety of advanced options are provided if one wishes fine grained control
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as opposed to automatic settings. If advanced options are used then auto-setting
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of parameters get disabled. The various advanced options are discussed below.
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Chunk-level Deduplication
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-------------------------
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Attempt Polynomial fingerprinting based deduplication on a per-chunk basis:
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pcompress -D ...
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Perform Delta Encoding in addition to Identical Dedup:
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pcompress -E ... - This also implies '-D'. This performs Delta Compression
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between 2 blocks if they are 40% to 60% similar. The
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similarity %age is selected based on the dedupe block
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size to balance performance and effectiveness.
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pcompress -EE .. - This causes Delta Compression to happen if 2 blocks are
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at least 40% similar regardless of block size. This can
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effect greater final compression ratio at the cost of
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higher processing overhead.
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-F Perform Fixed Block Deduplication. This is faster than fingerprinting
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based content-aware deduplication in some cases. However this is mostly
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usable for disk dumps especially virtual machine images. This generally
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gives lower dedupe ratio than content-aware dedupe (-D) and does not
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support delta compression.
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Global Deduplication
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--------------------
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-G This flag enables Global Deduplication. This makes pcompress maintain an
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in-memory index to lookup cryptographic block hashes for duplicates. Once
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a duplicate is found it is replaced with a reference to the original block.
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This allows detecting and eliminating duplicate blocks across the entire
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dataset. In contrast using only '-D' or '-F' flags does deduplication only
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within the chunk but uses less memory and is much faster than Global Dedupe.
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The '-G' flag can be combined with either '-D' or '-F' flags to indicate
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rabin chunking or fixed chunking respectively. If these flags are not
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specified then the default is to assume rabin chunking via '-D'.
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All other Dedupe flags have the same meanings in this context.
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Delta Encoding is not supported with Global Deduplication at this time. The
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in-memory hashtable index can use upto 75% of free RAM depending on the size
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of the dataset. In Pipe mode the index will always use 75% of free RAM since
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the dataset size is not known. This is the simple full block index mode. If
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the available RAM is not enough to hold all block checksums then older block
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entries are discarded automatically from the matching hash slots.
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If pipe mode is not used and the given dataset is a file then Pcompress
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checks whether the index size will exceed three times of 75% of the available
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free RAM. In such a case it automatically switches to a Segmented Deduplication
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mode. Here data is first split into blocks as above. Then upto 2048 blocks are
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grouped together to form a larger segment. The individual block hashes for a
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segment are stored on a tempfile on disk. A few min-values hashes are then
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computed from the block hashes of the segment which are then loaded into the
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index. These hashes are used to detect segments that are approximately similar
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to each other. Once found the block hashes of the matching segments are loaded
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from the temp file and actual deduplication is performed. This allows the
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in-memory index size to be approximately 0.0025% of the total dataset size and
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requires very few disk reads for every 2048 blocks processed.
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In pipe mode Global Deduplication always uses a segmented similarity based
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index. It allows efficient network transfer of large data.
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-B <0..5>
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Specify an average Dedupe block size. 0 - 2K, 1 - 4K, 2 - 8K ... 5 - 64K.
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Default deduplication block size is 4KB for Global Deduplication and 2KB
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otherwise.
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-B 0
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This uses blocks as small as 2KB for deduplication. This option can be
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used for datasets of a few GBs to a few hundred TBs in size depending on
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available RAM.
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-L Enable LZP pre-compression. This improves compression ratio of all
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algorithms with some extra CPU and very low RAM overhead. Using
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delta encoding in conjunction with this may not always be beneficial.
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However Adaptive Delta Encoding is beneficial along with this.
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-P Enable Adaptive Delta Encoding. It can improve compresion ratio further
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for data containing tables of numerical values especially if those are
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in an arithmetic series. In this implementation basic Delta Encoding is
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combined with Run-Length encoding and Matrix transpose
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NOTE - Both -L and -P can be used together to give maximum benefit on most
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datasets.
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-x Perform Dispack Encoding. This is useful to translate x86 call and jmp
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relative offsets to absolute values which compress better. The given
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chunk is split into 32KB blocks and some heuristics are used per block
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to identify whether it represents x86 instruction stream or not. This
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works only when archiving.
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-j Enable PackJPG processing for Jpeg files. This works only when archiving.
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-M Display memory allocator statistics.
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-C Display compression statistics.
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-CC Display compression statistics and print the offset and length of each
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variable length dedupe block if variable block deduplication is being
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used. This has no effect for fixed block deduplication.
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Environment Variables
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=====================
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Set ALLOCATOR_BYPASS=1 in the environment to avoid using the the built-in
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allocator. Due to the the way it rounds up an allocation request to the nearest
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slab the built-in allocator can allocate extra unused memory. In addition you
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may want to use a different allocator in your environment.
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The variable PCOMPRESS_INDEX_MEM can be set to limit memory used by the Global
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Deduplication Index. The number specified is in multiples of a megabyte.
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The variable PCOMPRESS_CACHE_DIR can point to a directory where some temporary
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files relating to the Global Deduplication process can be stored. This for example
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can be a directory on a Solid State Drive to speed up Global Deduplication. The
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space used in this directory is proportional to the size of the dataset being
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processed and is slightly more than 8KB for every 1MB of data.
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The default checksum used for block hashes during Global Deduplication is SHA256.
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However this can be changed by setting the PCOMPRESS_CHUNK_HASH_GLOBAL environment
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variable. The list of allowed checksums for this is:
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SHA256 , SHA512
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KECCAK256, KECCAK512
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BLAKE256 , BLAKE512
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SKEIN256 , SKEIN512
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Even though SKEIN is not supported as a chunk checksum (not deemed necessary
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because BLAKE2 is available) it can be used as a dedupe block checksum. One may
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ask why? The reasoning is we depend on hashes to find duplicate blocks. Now SHA256
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is the default because it is known to be robust and unbroken till date. Proven as
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yet in the field. However one may want a faster alternative so we have choices
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from the NIST SHA3 finalists in the form of SKEIN and BLAKE which are neck to
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neck with SKEIN getting an edge. SKEIN and BLAKE have seen extensive cryptanalysis
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in the intervening years and are unbroken with only marginal theoretical issues
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determined. BLAKE2 is a derivative of BLAKE and is tremendously fast but has not
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seen much specific cryptanalysis as yet, even though it is not new but just a
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performance optimized derivate. So cryptanalysis that applies to BLAKE should
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also apply and justify BLAKE2. However the paranoid may well trust SKEIN a bit
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more than BLAKE2 and SKEIN while not being as fast as BLAKE2 is still a lot faster
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than SHA2.
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Examples
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========
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Archive contents of directory /usr/include into usr.pz. Default chunk or per-thread
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segment size is 8MB and default compression level is 6.
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pcompress -a /usr/include usr
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Archive the given listr of files into file.pz and max compresion level and all features
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enabled. A maximum chunk size of 20MB is used. Also use verbose mode which lists each
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file as it is processed.
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pcompress -a -v -l14 -s20m file1 file2 file3 file
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Simple compress "file.tar" using zlib(gzip) algorithm. Default chunk or per-thread
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segment size is 8MB and default compression level is 6. Output file created will be
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file.tar.pz
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pcompress -c zlib file.tar
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Simple compress "file.tar" using zlib(gzip) algorithm with output file file.compressed.pz
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pcompress -c zlib file.tar file.compressed
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Compress "file.tar" using Zlib and per-thread chunk or segment size of 10MB and
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Compression level 9. Compressed output is sent to stdout using '-' which is then
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redirected to a file.
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pcompress -c zlib -l9 -s10m file.tar - > /path/to/compress_file.tar.pz
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It is possible for a single chunk to span the entire file if enough RAM is
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available. However for adaptive modes to be effective for large files, especially
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multi-file archives splitting into chunks is required so that best compression
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algorithm can be selected for textual and binary portions.
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Pre-Processing Algorithms
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=========================
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As can be seen above a multitude of pre-processing algorithms are available that
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provide further compression effectiveness beyond what the usual compression
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algorithms can achieve by themselves. These are summarized below:
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1) Deduplication : Per-Chunk (or per-segment) deduplication based on Rabin
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fingerprinting.
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2) Delta Compression : A similarity based (minhash) comparison of Rabin blocks.
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Two blocks at least 60% similar with each other are diffed
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using bsdiff.
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3) LZP : LZ Prediction is a variant of LZ77 that replaces repeating
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runs of text with shorter codes.
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4) Adaptive Delta : This is a simple form of Delta Encoding where arithmetic
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progressions are detected in the data stream and
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collapsed via Run-Length encoding.
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4) Matrix Transpose : This is used automatically in Delta Encoding and
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Deduplication. This attempts to transpose columnar
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|
repeating sequences of bytes into row-wise sequences so
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that compression algorithms can work better.
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Memory Usage
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============
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As can be seen from above memory usage can vary greatly based on compression/
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|
pre-processing algorithms and chunk size. A variety of configurations are possible
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|
depending on resource availability in the system.
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|
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|
The minimum possible meaningful settings while still giving about 50% compression
|
|
ratio and very high speed is with the LZFX algorithm with 1MB chunk size and 2
|
|
threads:
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pcompress -c lzfx -l2 -s1m -t2 <file>
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This uses about 6MB of physical RAM (RSS). Earlier versions of the utility before
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|
the 0.9 release comsumed much more memory. This was improved in the later versions.
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When using Linux the virtual memory consumption may appear to be very high but it
|
|
is just address space usage rather than actual RAM and should be ignored. It is only
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|
the RSS that matters. This is a result of the memory arena mechanism in Glibc that
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|
improves malloc() performance for multi-threaded applications.
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