bzip2.txt

絲路server源碼 Silk Road server source

              These  flags  are  redundant  in versions 0.9.5 and
              above.  They provided some coarse control over  the
              behaviour  of the sorting algorithm in earlier ver-
              sions, which was sometimes useful.  0.9.5 and above
              have  an  improved  algorithm  which  renders these
              flags irrelevant.


MEMORY MANAGEMENT
       bzip2 compresses large files in blocks.   The  block  size
       affects  both  the  compression  ratio  achieved,  and the
       amount of memory needed for compression and decompression.
       The  flags  -1  through  -9  specify  the block size to be
       100,000 bytes through 900,000 bytes (the default)  respec-
       tively.   At  decompression  time, the block size used for
       compression is read from  the  header  of  the  compressed
       file, and bunzip2 then allocates itself just enough memory
       to decompress the file.  Since block sizes are  stored  in
       compressed  files,  it follows that the flags -1 to -9 are
       irrelevant to and so ignored during decompression.

       Compression and decompression requirements, in bytes,  can
       be estimated as:

              Compression:   400k + ( 8 x block size )

              Decompression: 100k + ( 4 x block size ), or
                             100k + ( 2.5 x block size )

       Larger  block  sizes  give  rapidly  diminishing  marginal
       returns.  Most of the compression comes from the first two
       or  three hundred k of block size, a fact worth bearing in
       mind when using bzip2  on  small  machines.   It  is  also
       important  to  appreciate  that  the  decompression memory
       requirement is set at compression time by  the  choice  of
       block size.

       For  files  compressed  with  the default 900k block size,
       bunzip2 will require about 3700 kbytes to decompress.   To
       support decompression of any file on a 4 megabyte machine,
       bunzip2 has an option to  decompress  using  approximately
       half this amount of memory, about 2300 kbytes.  Decompres-
       sion speed is also halved, so you should use  this  option
       only where necessary.  The relevant flag is -s.

       In general, try and use the largest block size memory con-
       straints  allow,  since  that  maximises  the  compression
       achieved.   Compression and decompression speed are virtu-
       ally unaffected by block size.

       Another significant point applies to files which fit in  a
       single  block  --  that  means  most files you'd encounter
       using a large block  size.   The  amount  of  real  memory
       touched is proportional to the size of the file, since the
       file is smaller than a block.  For example, compressing  a
       file  20,000  bytes  long  with the flag -9 will cause the
       compressor to allocate around 7600k of  memory,  but  only
       touch 400k + 20000 * 8 = 560 kbytes of it.  Similarly, the
       decompressor will allocate 3700k but  only  touch  100k  +
       20000 * 4 = 180 kbytes.

       Here  is a table which summarises the maximum memory usage
       for different block sizes.  Also  recorded  is  the  total
       compressed  size for 14 files of the Calgary Text Compres-
       sion Corpus totalling 3,141,622 bytes.  This column  gives
       some  feel  for  how  compression  varies with block size.
       These figures tend to understate the advantage  of  larger
       block  sizes  for  larger files, since the Corpus is domi-
       nated by smaller files.

                  Compress   Decompress   Decompress   Corpus
           Flag     usage      usage       -s usage     Size

            -1      1200k       500k         350k      914704
            -2      2000k       900k         600k      877703
            -3      2800k      1300k         850k      860338
            -4      3600k      1700k        1100k      846899
            -5      4400k      2100k        1350k      845160
            -6      5200k      2500k        1600k      838626
            -7      6100k      2900k        1850k      834096
            -8      6800k      3300k        2100k      828642
            -9      7600k      3700k        2350k      828642


RECOVERING DATA FROM DAMAGED FILES
       bzip2 compresses files in blocks, usually 900kbytes  long.
       Each block is handled independently.  If a media or trans-
       mission error causes a multi-block  .bz2  file  to  become
       damaged,  it  may  be  possible  to  recover data from the
       undamaged blocks in the file.

       The compressed representation of each block  is  delimited
       by  a  48-bit pattern, which makes it possible to find the
       block boundaries with reasonable  certainty.   Each  block
       also  carries its own 32-bit CRC, so damaged blocks can be
       distinguished from undamaged ones.

       bzip2recover is a  simple  program  whose  purpose  is  to
       search  for blocks in .bz2 files, and write each block out
       into its own .bz2 file.  You can then use bzip2 -t to test
       the integrity of the resulting files, and decompress those
       which are undamaged.

       bzip2recover takes a single argument, the name of the dam-
       aged    file,    and    writes    a    number   of   files
       "rec00001file.bz2",  "rec00002file.bz2",  etc,  containing
       the   extracted   blocks.   The   output   filenames   are
       designed  so  that the use of wildcards in subsequent pro-
       cessing  -- for example, "bzip2 -dc  rec*file.bz2 > recov-
       ered_data" -- processes the files in the correct order.

       bzip2recover should be of most use dealing with large .bz2
       files,  as  these will contain many blocks.  It is clearly
       futile to use it on damaged single-block  files,  since  a
       damaged  block  cannot  be recovered.  If you wish to min-
       imise any potential data loss through media  or  transmis-
       sion errors, you might consider compressing with a smaller
       block size.


PERFORMANCE NOTES
       The sorting phase of compression gathers together  similar
       strings  in  the  file.  Because of this, files containing
       very long runs of  repeated  symbols,  like  "aabaabaabaab
       ..."   (repeated  several hundred times) may compress more
       slowly than normal.  Versions 0.9.5 and  above  fare  much
       better  than previous versions in this respect.  The ratio
       between worst-case and average-case compression time is in
       the  region  of  10:1.  For previous versions, this figure
       was more like 100:1.  You can use the -vvvv option to mon-
       itor progress in great detail, if you want.

       Decompression speed is unaffected by these phenomena.

       bzip2  usually  allocates  several  megabytes of memory to
       operate in, and then charges all over it in a fairly  ran-
       dom  fashion.   This means that performance, both for com-
       pressing and decompressing, is largely determined  by  the
       speed  at  which  your  machine  can service cache misses.
       Because of this, small changes to the code to  reduce  the
       miss  rate  have  been observed to give disproportionately
       large performance improvements.  I imagine bzip2 will per-
       form best on machines with very large caches.


CAVEATS
       I/O  error  messages  are not as helpful as they could be.
       bzip2 tries hard to detect I/O errors  and  exit  cleanly,
       but  the  details  of  what  the problem is sometimes seem
       rather misleading.

       This manual page pertains to version 1.0.3 of bzip2.  Com-
       pressed  data created by this version is entirely forwards
       and  backwards  compatible  with   the   previous   public
       releases,  versions 0.1pl2, 0.9.0, 0.9.5, 1.0.0, 1.0.1 and
       1.0.2, but with the following exception: 0.9.0  and  above
       can  correctly decompress multiple concatenated compressed
       files.  0.1pl2 cannot do this; it will stop  after  decom-
       pressing just the first file in the stream.

       bzip2recover  versions prior to 1.0.2 used 32-bit integers
       to represent bit positions in compressed  files,  so  they
       could  not handle compressed files more than 512 megabytes
       long.  Versions 1.0.2 and above use 64-bit  ints  on  some
       platforms  which  support them (GNU supported targets, and
       Windows).  To establish whether or  not  bzip2recover  was
       built  with  such  a limitation, run it without arguments.
       In any event you can build yourself an  unlimited  version
       if  you  can  recompile  it  with MaybeUInt64 set to be an
       unsigned 64-bit integer.


AUTHOR
       Julian Seward, jsewardbzip.org.

       http://www.bzip.org

       The ideas embodied in bzip2 are due to (at least) the fol-
       lowing  people: Michael Burrows and David Wheeler (for the
       block sorting transformation), David Wheeler  (again,  for
       the Huffman coder), Peter Fenwick (for the structured cod-
       ing model in the original bzip, and many refinements), and
       Alistair  Moffat,  Radford  Neal  and  Ian Witten (for the
       arithmetic  coder  in  the  original  bzip).   I  am  much
       indebted for their help, support and advice.  See the man-
       ual in the source distribution for pointers to sources  of
       documentation.  Christian von Roques encouraged me to look
       for faster sorting algorithms, so as to speed up  compres-
       sion.  Bela Lubkin encouraged me to improve the worst-case
       compression performance.  Donna Robinson XMLised the docu-
       mentation.   The bz* scripts are derived from those of GNU
       gzip.  Many people sent patches, helped  with  portability
       problems,  lent  machines,  gave advice and were generally
       helpful.