=head1 NAME

nbdkit-noparallel-filter - nbdkit noparallel filter

=head1 SYNOPSIS

 nbdkit --filter=noparallel plugin [serialize=MODE] [plugin-args...]

=head1 DESCRIPTION

C<nbdkit-noparallel-filter> is a filter that intentionally disables
parallelism in handling requests from clients. It is mainly useful for
evaluating timing differences between various levels of
parallelism. It can also be used as a way to work around any bugs in a
plugin's claimed level of parallel support, without recompiling the
plugin, or to ease efforts when connecting with a client that can
batch up several requests but is not prepared to handle out-of-order
replies.

=head1 PARAMETERS

=over 4

=item B<serialize=requests>

=item B<serialize=all-requests>

=item B<serialize=connections>

Optional, controls how much serialization the filter will
enforce. Mode B<requests> (default) prevents a single client from
having more than one in-flight request, but does not prevent parallel
requests from a second connection (if the plugin supports that). Mode
B<all-requests> is stricter, enforcing that at most one request
(regardless of connection) will be active, but does not prevent
parallel connections (if the plugin supports that). Mode
B<connections> is strictest, where there can be at most one client at
a time, and where the server will no longer advertise
C<NBD_FLAG_MULTI_CONN> to clients.

=back

=head1 EXAMPLES

Serve the file F<disk.img>, but disallow out-of-order transaction
completion to a given client:

 nbdkit --filter=noparallel file disk.img

Serve the file F<disk.img>, but allowing only one client at a time:

 nbdkit --filter=noparallel file serialize=connections disk.img

=head1 FILES

=over 4

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

The filter.

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

=back

=head1 VERSION

C<nbdkit-noparallel-filter> first appeared in nbdkit 1.14.

=head1 SEE ALSO

L<nbdkit(1)>,
L<nbdkit-file-plugin(1)>,
L<nbdkit-filter(3)>,
L<nbdkit-fua-filter(1)>,
L<nbdkit-nocache-filter(1)>,
L<nbdkit-noextents-filter(1)>,
L<nbdkit-nozero-filter(1)>.

=head1 AUTHORS

Eric Blake

=head1 COPYRIGHT

Copyright (C) 2018-2019 Red Hat Inc.
