Re: [Libguestfs] [nbdkit PATCH v3 15/16] multi-conn: New filter

Implement a TODO item of emulating multi-connection consistency via multiple plugin flush calls to allow a client to assume that a flush on a single connection is good enough. This also gives us some fine-tuning over whether to advertise the bit, including some setups that are unsafe but may be useful in timing tests. Testing is interesting: I used the sh plugin to implement a server that intentionally keeps a per-connection cache. Note that this filter assumes that multiple connections will still share the same data (other than caching effects); effects are not guaranteed when trying to mix it with more exotic plugins like info that violate that premise. --- filters/cache/nbdkit-cache-filter.pod | 5 +- filters/fua/nbdkit-fua-filter.pod | 7 + .../multi-conn/nbdkit-multi-conn-filter.pod | 178 +++++++ filters/nocache/nbdkit-nocache-filter.pod | 1 + filters/noextents/nbdkit-noextents-filter.pod | 1 + .../noparallel/nbdkit-noparallel-filter.pod | 1 + filters/nozero/nbdkit-nozero-filter.pod | 1 + configure.ac | 4 +- filters/multi-conn/Makefile.am | 68 +++ tests/Makefile.am | 9 + filters/multi-conn/multi-conn.c | 435 ++++++++++++++++++ tests/test-multi-conn-plugin.sh | 122 +++++ tests/test-multi-conn.sh | 293 ++++++++++++ TODO | 7 - 14 files changed, 1123 insertions(+), 9 deletions(-) create mode 100644 filters/multi-conn/nbdkit-multi-conn-filter.pod create mode 100644 filters/multi-conn/Makefile.am create mode 100644 filters/multi-conn/multi-conn.c create mode 100755 tests/test-multi-conn-plugin.sh create mode 100755 tests/test-multi-conn.sh diff --git a/filters/cache/nbdkit-cache-filter.pod b/filters/cache/nbdkit-cache-filter.pod index 34fd0b29..482a9c55 100644 --- a/filters/cache/nbdkit-cache-filter.pod +++ b/filters/cache/nbdkit-cache-filter.pod @@ -41,7 +41,9 @@ an explicit flush is done by the client. This is the default caching mode, and is safe if your client issues flush requests correctly (which is true for modern Linux and other -well-written NBD clients). +well-written NBD clients). Note that this mode is able to advertise +multi-connection consistency even without the use of +L<nbdkit-multi-conn-filter(1)>.

=item B<cache=writethrough> @@ -162,6 +164,7 @@ L<nbdkit(1)>, L<nbdkit-file-plugin(1)>, L<nbdkit-cacheextents-filter(1)>, L<nbdkit-readahead-filter(1)>, +L<nbdkit-multi-conn-filter(1)>, L<nbdkit-filter(3)>, L<qemu-img(1)>.

diff --git a/filters/fua/nbdkit-fua-filter.pod b/filters/fua/nbdkit-fua-filter.pod index 0f9b9744..9b0d84b5 100644 --- a/filters/fua/nbdkit-fua-filter.pod +++ b/filters/fua/nbdkit-fua-filter.pod @@ -16,6 +16,12 @@ This filter can be used to disable FUA and flush requests for speed server fallbacks, and for evaluating timing differences between proper use of FUA compared to a full flush. +Note that by default, the NBD protocol does not guarantee that the use +of FUA from one connection will be visible from another connection +unless the server advertised NBD_FLAG_MULTI_CONN. You may wish to +combine this filter with L<nbdkit-multi-conn-filter(1)> if you plan on +making multiple connections to the plugin.

=head1 PARAMETERS The C<fuamode> parameter is optional and controls which mode the @@ -137,6 +143,7 @@ L<nbdkit-file-plugin(1)>, L<nbdkit-filter(3)>, L<nbdkit-blocksize-filter(1)>, L<nbdkit-log-filter(1)>, +L<nbdkit-multi-conn-filter(1)>, L<nbdkit-nocache-filter(1)>, L<nbdkit-noextents-filter(1)>, L<nbdkit-noparallel-filter(1)>, diff --git a/filters/multi-conn/nbdkit-multi-conn-filter.pod b/filters/multi-conn/nbdkit-multi-conn-filter.pod new file mode 100644 index 00000000..c8f334b3 --- /dev/null +++ b/filters/multi-conn/nbdkit-multi-conn-filter.pod