NAME

       nbdkit-nocache-filter - nbdkit nocache filter

SYNOPSIS

        nbdkit --filter=nocache plugin [cachemode=MODE] [plugin-args...]

DESCRIPTION

       "nbdkit-nocache-filter" is a filter that intentionally disables efficient handling of
       advisory client cache requests across the NBD protocol.  It is mainly useful for
       evaluating timing differences to determine the impact of caching requests.

       Note that the effects of this filter (in crippling handling of client cache requests) is
       somewhat orthogonal from that of the nbdkit-cache-filter(1) (adding local caching of
       client read/write requests); the two filters can be run together to experiment with
       timings.

PARAMETERS

       cachemode=none
       cachemode=emulate
       cachemode=nop
           Optional, controls which mode the filter will use.  Mode none (default) means that
           cache support is not advertised to the client. Mode emulate means that cache support
           is emulated by the filter using the plugin's "pread" callback, regardless of whether
           the plugin itself implemented the "cache" callback. Mode nop means that cache requests
           are always accepted and immediately ignored, rather than having any actual impact.

EXAMPLES

       Serve the file disk.img, but prevent "NBD_CMD_CACHE" requests altogether, to get a
       baseline timing of behavior when the client is unable to make cache requests:

        nbdkit --filter=nocache file disk.img

       Serve the file disk.img, but with cache requests silently ignored, rather than being
       forwarded on to the file plugin (which attempts to use posix_fadvise(3)), to compare
       against the timings without the filter and determine whether the file plugin caching was
       worthwhile:

        nbdkit --filter=nocache file disk.img cachemode=nop

FILES

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

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

VERSION

       "nbdkit-nocache-filter" first appeared in nbdkit 1.14.

SEE ALSO

       nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3), nbdkit-cache-filter(1),
       nbdkit-fua-filter(1), nbdkit-multi-conn-filter(1), nbdkit-noextents-filter(1),
       nbdkit-noparallel-filter(1), nbdkit-nozero-filter(1).

AUTHORS

       Eric Blake

COPYRIGHT

       Copyright (C) 2019 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.