NAME

       nbdkit-truncate-filter - change the size of plugins

SYNOPSIS

        nbdkit --filter=truncate plugin [truncate=SIZE]
                                        [round-up=N] [round-down=N]

DESCRIPTION

       "nbdkit-truncate-filter" is a filter that changes the size of the underlying plugin.  It can:

       •   Make the plugin smaller (truncate it).  Use the "truncate=SIZE" parameter to set the smaller size.

       •   Make  the  plugin  larger (the additional bytes read back as zeroes).  Use "truncate=SIZE" to set the
           larger size.

       •   Round the size of the plugin up or down to the next multiple of  "N".   Use  either  "round-up=N"  or
           "round-down=N".

       A  common  use  for  this  filter is to handle NBD clients which have a problem dealing with device sizes
       which are not a multiple of 512 bytes.  Use "round-up=512" to round the size up to the next  multiple  of
       512 bytes.  If the size is already a multiple of 512 bytes then this has no effect.

PARAMETERS

       truncate=SIZE
           Set  the absolute size in bytes of the apparent device.  This may be smaller or larger or the same as
           the underlying plugin.

           If the size is larger than the underlying plugin, reading the extra space returns zeroes.  Writes are
           also permitted to the extra space, but you must only write zeroes (any  attempts  to  write  non-zero
           bytes will return an error back to the client).

           This parameter is optional.

       round-up=N
           Round the size up to the next multiple of "N" bytes.  If the size of the underlying plugin is already
           a multiple of "N" bytes, this has no effect.

           This  option  is  useful  when  combined  with  other  filters  (such  as nbdkit-blocksize-filter(1),
           nbdkit-cache-filter(1),  or  nbdkit-cow-filter(1))  that  normally  round  down  due  to   a   larger
           granularity,  in  order  to  access  the  last  few  bytes of a file that would otherwise be rendered
           inaccessible.

           This parameter is optional.

       round-down=N
           Round the size down to a multiple of "N" bytes.  If the size of the underlying plugin  is  already  a
           multiple of "N" bytes, this has no effect.

           This parameter is optional.

EXAMPLES

       Serve  a  file  as if it were a block device that insists on 4k alignment, while still allowing access to
       any unaligned bytes at the end of the file:

        nbdkit --filter=blocksize --filter=truncate file /path/to/file \
        minblock=4k round-up=4k

FILES

       $filterdir/nbdkit-truncate-filter.so
           The filter.

           Use "nbdkit --dump-config" to find the location of $filterdir.

VERSION

       "nbdkit-truncate-filter" first appeared in nbdkit 1.6.

SEE ALSO

       nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3),  nbdkit-blocksize-filter(1),  nbdkit-cache-filter(1),
       nbdkit-cow-filter(1), nbdkit-offset-filter(1), nbdkit-partition-filter(1).

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2018 Red Hat Inc.

LICENSE

       Redistribution  and  use in source and binary forms, with or without modification, are permitted provided
       that the following conditions are met:

       •   Redistributions of source code must retain the above copyright notice, this list  of  conditions  and
           the following disclaimer.

       •   Redistributions in binary form must reproduce the above copyright notice, this list of conditions and
           the following disclaimer in the documentation and/or other materials provided with the distribution.

       •   Neither  the  name  of  Red  Hat  nor the names of its contributors may be used to endorse or promote
           products derived from this software without specific prior written permission.

       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS  OR  IMPLIED  WARRANTIES,
       INCLUDING,  BUT  NOT  LIMITED  TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE  FOR  ANY  DIRECT,  INDIRECT,
       INCIDENTAL,  SPECIAL,  EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
       ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT  LIABILITY,  OR  TORT  (INCLUDING  NEGLIGENCE  OR
       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
       DAMAGE.

nbdkit-1.16.2                                      2020-01-28                          nbdkit-truncate-filter(1)