Re: [Libguestfs] [nbdkit PATCH v3 16/16] multi-conn: Knob to limit consistency emulation by export name

Subject: multi-conn: Knob to limit consistency emulation by export name We can't enable the knob by default since many nbdkit plugins don't care about export name, but it is certainly more efficient to limit consistency to just clients visiting the same export. Thankfully, NBD_CMD_FLUSH does not take size and offset, and therefore is safe (although wasting CPU cycles) to use even when a different export has a different size. Done by adding another layer of indirection: all connections are combined into a group (a singleton when the knob is off, or by name when the knob is on), moving fast dirty tracking into that group.

--- .../multi-conn/nbdkit-multi-conn-filter.pod | 27 +++- tests/Makefile.am | 2 + filters/multi-conn/multi-conn.c | 118 +++++++++++++++--- tests/test-multi-conn-name.sh | 88 +++++++++++++ tests/test-multi-conn-plugin.sh | 63 ++++++---- 5 files changed, 255 insertions(+), 43 deletions(-) create mode 100755 tests/test-multi-conn-name.sh diff --git a/filters/multi-conn/nbdkit-multi-conn-filter.pod b/filters/multi-conn/nbdkit-multi-conn-filter.pod index c8f334b3..efeba570 100644 --- a/filters/multi-conn/nbdkit-multi-conn-filter.pod +++ b/filters/multi-conn/nbdkit-multi-conn-filter.pod @@ -5,7 +5,7 @@ nbdkit-multi-conn-filter - nbdkit multi-conn filter =head1 SYNOPSIS nbdkit --filter=multi-conn plugin [multi-conn-mode=MODE] \ - [multi-conn-track-dirty=LEVEL] [plugin-args...] + [multi-conn-track-dirty=LEVEL] [multi-conn-exportname=BOOL] [plugin-args...] =head1 DESCRIPTION @@ -26,7 +26,8 @@ it also has additional modes useful for evaluating performance and correctness of client and plugin multi-conn behaviors. This filter assumes that multiple connections to a plugin will eventually share data, other than any caching effects; it is not suitable for use with -a plugin that produces completely independent data per connection. +a plugin that produces completely independent data per connection from +the same export name.

Additional control over the behavior of client flush commands is possible by combining this filter with L<nbdkit-fua-filter(1)>. Note @@ -125,6 +126,27 @@ B<multi-conn-mode=emulate>, a client which disregards NBD_FLAG_MULTI_CONN by flushing on each connection itself results in a quadratic number of flush operations on the plugin. +=item B<multi-conn-exportname=false> + +The exportname switch defaults to false for safety, and causes the +filter to flush across all active connections regardless of the export +name in use by that connection when doing emulation. However, when a +plugin supports distinct data according to export name, this behavior +will penalize the performance of clients visiting an unrelated export +by spending time on replicated flush operations not actually relevant +to that export. + +=item B<multi-conn-exportname=true> + +Setting the exportname switch to true causes the filter to only +synchronize flushes to connections visiting the same export name. +This avoids penalizing clients visiting an unrelated export name (such +as L<nbdkit-file-plugin(1)> in B<dir=> mode), but is unsafe when used +with a plugin that serves shared content across all connections +regardless of the export name requested by the client, if that plugin +is not already multi-conn consistent (such as +L<nbdkit-vddk-plugin(1)>).