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)) 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:

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           conditions and the following disclaimer.

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           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.

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