trusty (1) buffer.1.gz

NAME

       buffer - very fast reblocking program

SYNTAX

       buffer  [-S  size]  [-b blocks] [-s size] [-z size] [-m size] [-p percentage] [-u microseconds] [-B] [-t]
       [-Z] [-i filename] [-o filename] [-d]

OPTIONS

       -i filename
            Use the given file as the input file.  The default is stdin.

       -o filename
            Use the given file as the output file.  The default is stdout.

       -S size
            After every chunk of this size has been written, print out how much has been written  so  far.  Also
            prints the total throughput.  By default this is not set.

       -s size
            Size  in bytes of each block.  The default blocksize is 10k to match the normal output of the tar(1)
            program.

       -z size
            Combines the -S and -s flags.

       -b blocks
            Number of blocks to allocate to shared memory circular buffer.  Defaults to the number  required  to
            fill up the shared memory requested.

       -m size
            Maximum  size  of  the  shared  memory  chunk  to  allocate  for the circular queue. Defaults to one
            megabyte.

       -p percentage
            Only start a write when the given percentage of the internal queue is full.  A percentage around  75
            often proves best. Defaults to zero.

       -u microseconds
            After every write pause for this many microseconds.  Defaults to zero.  (Surprisingly a small sleep,
            100 usecs, after each write can greatly enhance throughput on some drives.)

       -B   Force each block written to be padded out to the  blocksize.   This  is  needed  by  some  tape  and
            cartridge drives.  Defaults to unpadded.  This only affects the last block written.

       -t   On exiting print to stderr a brief message showing the total number of bytes written.

       -Z   If  reading/writing  directly  to  a  character  device (like a tape drive) then after each gigabyte
            perform an lseek to the start of the file.  Use this flag with extreme care.  It can only be used on
            devices  where  an  lseek does not rewind the tape but does reset the kernels position flags.  It is
            used to allow more than 2 gigabytes to be written.

       -d   Print debug information to stderr during operation.

       Sizes are a number with an optional trailing character.   A 'b' multiplies the size by 512, a 'k' by 1024
       and an 'm' by a meg.

DESCRIPTION

       Buffer  reads  from  standard  input  reblocking to the given blocksize and writes each block to standard
       output.

       Internally buffer is a pair of processes communicating via a large circular queue held in shared  memory.
       The  reader  process  only  has  to block when the queue is full and the writer process when the queue is
       empty.  Buffer is designed to try and keep the writer side continuously busy so that it can  stream  when
       writing  to  tape  drives.   When  used to write tapes with an intervening network buffer can result in a
       considerable increase in throughput.

       The default settings for buffer are normally good enough.  If you are a heavy tape user then it is  worth
       your  while trying out various different combinations of options.  In particular running a buffer at both
       ends of the pipe can provide a substantial increase (see last example below).

EXAMPLES

       $ buffer < /etc/termcap > /dev/rst8

       $ tar cf - . | rsh somehost 'buffer > /dev/rst8'

       $ dump fu - | rsh somehost 'buffer -s 16k > /dev/nrst8'
       $ tar cf - . | buffer |
          rsh somehost 'buffer -S 500K -p 75 > /dev/rst0'

BUFFER LIMITS

       There are 2 limits in buffer affecting the maximum total buffer size: The maximum number of blocks (2048)
       and the maximum size of a block (512kB). This results in a maximum total buffer size of 1GB.

       Note  that  there  is also a kernel limit for the maximum size of a shared memory segment (used by buffer
       internally) which is typically much lower than the limits in buffer.  So if buffer  complains  about  not
       being able to create a shared memory segment, this limit might have to be raised using the command sysctl
       kernel.shmmax=XXX (see also sysctl(1) and proc(5)).

BUGS

       Internally, for printing purposes, buffer counts in terms of the number  of  kilobytes  output.   If  the
       blocksize you use is not a whole number of kilobytes then the numbers printed will be inaccurate.

THANKS

       Thanks to Kevin Twidle <kpt@doc.ic.ac.uk> for a lot of early suggestions and patches to make it work with
       non-tar/dump tapes to exabyte drives.

       Thanks to Andi  Karrer  <karrer@bernina.ethz.ch>,  Rumi  Zahir  <rumi@iis.ethz.ch>  and  Christoph  Wicki
       <wicki@iis.ethz.ch>  for  patches  to  make buffer work when trying to write single tape files of greater
       than 2 gigabytes.

COPYRIGHT

       Copyright (C) 1990, 1991 by Lee McLoughlin.

SEE ALSO

       dd(1), tar(1), rsh(1)

                                                   14 May 1990                                         BUFFER(1)