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-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
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