1=head1 NAME 2 3nbdkit-nocache-filter - nbdkit nocache filter 4 5=head1 SYNOPSIS 6 7 nbdkit --filter=nocache plugin [cachemode=MODE] [plugin-args...] 8 9=head1 DESCRIPTION 10 11C<nbdkit-nocache-filter> is a filter that intentionally disables 12efficient handling of advisory client cache requests across the NBD 13protocol. It is mainly useful for evaluating timing differences to 14determine the impact of caching requests. 15 16Note that the effects of this filter (in crippling handling of client 17cache requests) is somewhat orthogonal from that of the 18L<nbdkit-cache-filter(1)> (adding local caching of client read/write 19requests); the two filters can be run together to experiment with 20timings. 21 22=head1 PARAMETERS 23 24=over 4 25 26=item B<cachemode=none> 27 28=item B<cachemode=emulate> 29 30=item B<cachemode=nop> 31 32Optional, controls which mode the filter will use. Mode B<none> 33(default) means that cache support is not advertised to the 34client. Mode B<emulate> means that cache support is emulated by the 35filter using the plugin's C<pread> callback, regardless of whether the 36plugin itself implemented the C<cache> callback. Mode B<nop> means 37that cache requests are always accepted and immediately ignored, 38rather than having any actual impact. 39 40=back 41 42=head1 EXAMPLES 43 44Serve the file F<disk.img>, but prevent C<NBD_CMD_CACHE> requests 45altogether, to get a baseline timing of behavior when the client is 46unable to make cache requests: 47 48 nbdkit --filter=nocache file disk.img 49 50Serve the file F<disk.img>, but with cache requests silently ignored, 51rather than being forwarded on to the file plugin (which attempts to 52use L<posix_fadvise(3)>), to compare against the timings without the 53filter and determine whether the file plugin caching was worthwhile: 54 55 nbdkit --filter=nocache file disk.img cachemode=nop 56 57=head1 FILES 58 59=over 4 60 61=item F<$filterdir/nbdkit-nocache-filter.so> 62 63The filter. 64 65Use C<nbdkit --dump-config> to find the location of C<$filterdir>. 66 67=back 68 69=head1 VERSION 70 71C<nbdkit-nocache-filter> first appeared in nbdkit 1.14. 72 73=head1 SEE ALSO 74 75L<nbdkit(1)>, 76L<nbdkit-file-plugin(1)>, 77L<nbdkit-filter(3)>, 78L<nbdkit-cache-filter(1)>, 79L<nbdkit-fua-filter(1)>, 80L<nbdkit-noextents-filter(1)>, 81L<nbdkit-noparallel-filter(1)>, 82L<nbdkit-nozero-filter(1)>. 83 84=head1 AUTHORS 85 86Eric Blake 87 88=head1 COPYRIGHT 89 90Copyright (C) 2019 Red Hat Inc. 91