398 lines
20 KiB
Markdown
398 lines
20 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 a utility to do compression and decompression in parallel by
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splitting input data into chunks. It has a modular structure and includes
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support for multiple algorithms like LZMA, Bzip2, PPMD, etc, with SKEIN/
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SHA checksums for data integrity. It can also do Lempel-Ziv pre-compression
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(derived from libbsc) to improve compression ratios across the board. SSE
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optimizations for the bundled LZMA are included. It also implements
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Variable Block Deduplication and Delta Compression features based on a
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Semi-Rabin Fingerprinting scheme. Delta Compression is done via the widely
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popular bsdiff algorithm. Similarity is detected using a technique based
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on MinHashing. When doing Dedupe it attempts to merge adjacent non-
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duplicate block index entries into a single larger entry to reduce metadata.
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In addition to all these it can internally split chunks at rabin boundaries
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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. It also provides adaptive compression
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modes in which data analysis heuristics are used to identify near-optimal
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algorithms per chunk. Finally it supports 14 compression levels to allow
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for ultra compression parameters in some algorithms.
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Pcompress also supports encryption via AES and uses Scrypt from Tarsnap
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for Password Based Key generation. A unique key is generated per session
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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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Usage
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=====
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To compress a file:
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pcompress -c <algorithm> [-l <compress level>] [-s <chunk size>] <file> [<target file>]
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Where <algorithm> can be the folowing:
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lzfx - Very fast and small algorithm based on LZF.
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lz4 - Ultra fast, high-throughput algorithm reaching RAM B/W at level1.
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zlib - The base Zlib format compression (not Gzip).
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lzma - The LZMA (Lempel-Ziv Markov) algorithm from 7Zip.
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lzmaMt - Multithreaded version of LZMA. This is a faster version but
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uses more memory for the dictionary. Thread count is balanced
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between chunk processing threads and algorithm threads.
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bzip2 - Bzip2 Algorithm from libbzip2.
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ppmd - The PPMd algorithm excellent for textual data. PPMd requires
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at least 64MB X core-count more memory than the other modes.
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libbsc - A Block Sorting Compressor using the Burrows Wheeler Transform
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like Bzip2 but runs faster and gives better compression than
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Bzip2 (See: libbsc.com).
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adapt - Adaptive mode where ppmd or bzip2 will be used per chunk,
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depending on heuristics. If at least 50% of the input data is
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7-bit text then PPMd will be used otherwise Bzip2.
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adapt2 - Adaptive mode which includes ppmd and lzma. If at least 80% of
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the input data is 7-bit text then PPMd will be used otherwise
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LZMA. It has significantly more memory usage than adapt.
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none - No compression. This is only meaningful with -D and -E so Dedupe
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can be done for post-processing with an external utility.
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<chunk_size> - This can be in bytes or can use the following suffixes:
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g - Gigabyte, m - Megabyte, k - Kilobyte.
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Larger chunks produce better compression at the cost of memory.
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In case of Global Deduplication (see below) this chunk size is
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just a hint and may get automatically adjusted.
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<compress_level> - Can be a number from 0 meaning minimum and 14 meaning
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maximum compression.
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<target file> - Optional argument specifying the destination compressed
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file. The '.pz' extension is appended. If this is '-' then
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compressed output goes to stdout. If this argument is omitted then
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source filename is used with the extension '.pz' appended.
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NOTE: The option "libbsc" uses Ilya Grebnov's block sorting compression library
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from http://libbsc.com/ . It is only available if pcompress in built with
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that library. See INSTALL file for details.
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To decompress a file compressed using above command:
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pcompress -d <compressed file> <target file>
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<compressed file> can be '-' to indicate reading from stdin while write goes
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to <target file>
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To operate as a full pipe, read from stdin and write to stdout:
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pcompress -p ...
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Attempt Rabin 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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Number of threads can optionally be specified: -t <1 - 256 count>
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Other flags:
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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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'-S' <cksum>
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- Specify chunk checksum to use:
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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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'-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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'-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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Caveats:
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In some cases like LZMA with extreme compression levels and with '-L' and
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'-P' preprocessing enabled, this can result in lower compression as compared
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to using '-B 1'.
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For fast compression algorithms like LZ4 and Zlib this should always benefit.
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However please test on your sample data with your desired compression
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algorithm to verify the results.
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'-M' - Display memory allocator statistics
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'-C' - Display compression statistics
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Global Deduplication:
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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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Encryption flags:
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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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NOTE: When using pipe-mode via -p the only way to provide a password is to use '-w'.
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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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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.
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pcompress -c zlib file.tar
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Compress "file.tar" using bzip2 level 6, 64MB chunk size and use 4 threads. In
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addition perform identity deduplication and delta compression prior to compression.
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pcompress -D -E -c bzip2 -l6 -s64m -t4 file.tar
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Compress "file.tar" using zlib and also perform Global Deduplication. Default block
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size used for deduplication is 4KB. Also redirect the compressed output to stdout and
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send it to a compressed file at a different path.
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pcompress -G -c zlib -l9 -s10m file.tar - > /path/to/compress_file.tar.pz
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Perform the same as above but this time use a deduplication block size of 8KB.
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pcompress -G -c zlib -l9 -B2 -s10m file.tar - > /path/to/compress_file.tar.pz
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Compress "file.tar" using extreme compression mode of LZMA and a chunk size of
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of 1GB. Allow pcompress to detect the number of CPU cores and use as many threads.
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pcompress -c lzma -l14 -s1g file.tar
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Compress "file.tar" using lz4 at max compression with LZ-Prediction pre-processing
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and encryption enabled. Chunksize is 100M:
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pcompress -c lz4 -l3 -e -L -s100m file.tar
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Compression Algorithms
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======================
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LZFX - Ultra Fast, average compression. This algorithm is the fastest overall.
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Levels: 1 - 5
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LZ4 - Very Fast, better compression than LZFX.
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Levels: 1 - 3
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Zlib - Fast, better compression.
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Levels: 1 - 9
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Bzip2 - Slow, much better compression than Zlib.
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Levels: 1 - 9
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LZMA - Very slow. Extreme compression.
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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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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.
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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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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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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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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. Two
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blocks at least 60% similar with each other are diffed using
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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 collapsed
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via Run-Length encoding.
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4) Matrix Transpose : This is used automatically in Delta Encoding and Deduplication.
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This attempts to transpose columnar repeating sequences of
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bytes into row-wise sequences so that compression algorithms
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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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The minimum possible meaningful settings while still giving about 50% compression
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ratio and very high speed is with the LZFX algorithm with 1MB chunk size and 2
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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
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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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