=head1 NAME

nbdkit-truncate-filter - change the size of plugins

=head1 SYNOPSIS

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

=head1 DESCRIPTION

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

=over 4

=item *

Make the plugin smaller (truncate it).  Use the C<truncate=SIZE>
parameter to set the smaller size.

=item *

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

=item *

Round the size of the plugin up or down to the next multiple of C<N>.
Use either C<round-up=N> or C<round-down=N>.

=back

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 C<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.

=head1 PARAMETERS

=over 4

=item B<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.

=item B<round-up=>N

Round the size up to the next multiple of C<N> bytes.  If the size of
the underlying plugin is already a multiple of C<N> bytes, this has no
effect.

This option is useful when combined with other filters (such as
L<nbdkit-blocksize-filter(1)>, L<nbdkit-cache-filter(1)>, or
L<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.

=item B<round-down=>N

Round the size down to a multiple of C<N> bytes.  If the size of the
underlying plugin is already a multiple of C<N> bytes, this has no
effect.

This parameter is optional.

=back

=head1 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

=head1 FILES

=over 4

=item F<$filterdir/nbdkit-truncate-filter.so>

The filter.

Use C<nbdkit --dump-config> to find the location of C<$filterdir>.

=back

=head1 VERSION

C<nbdkit-truncate-filter> first appeared in nbdkit 1.6.

=head1 SEE ALSO

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

=head1 AUTHORS

Richard W.M. Jones

=head1 COPYRIGHT

Copyright (C) 2018 Red Hat Inc.
