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 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.