[Libguestfs] nbdkit design: varying the rate limit parameter

[Richard W.M. Jones]

Hi Eric,

We may have our first user (Tomas, CC'd) of the rate filter to limit
NBD connections.  As you recall it lets you do commands like:

  nbdkit --filter=rate memory size=64G rate=10M

to limit network bandwidth to 10 Mbps.  However a twist is that he
needs to vary the parameter while nbdkit is running in response to a
UI, and we haven't really thought about that before.

In the grand and noble tradition of qemu we don't actually systematize
parameters.  For the server they are just (key, value) strings.  (It's
even possible for filters to modify plugin parameters, thankfully
nothing does that.)

The only filter we have which does anything similar is the error
filter, where the error-file parameter lets you enable and disable the
filter at runtime based on the presence of a file.

  rm -f /tmp/inject
  nbdkit --filter=error file disk.img error-rate=100%  error-file=/tmp/inject
  sleep 60; touch /tmp/inject

I think our options are:

* One-time hack in the rate filter.

* One-time hack in the rate filter, but make it compatible with a
  possible future revision of parameter handling.  eg: If we decided
  that "rate:file" would in future mean "read ‘rate’ parameter from a
  file" then we could add this as a simple parameter now but
  systematize it in future.

* Systematize parameters.  This would require a great deal of work and
  likely another version of the API.  (I pushed a note into the TODO
  file for this.)

If we went all the way and allowed parameter values to change, would
this be triggered only by simple file changes, or would we want an
administration interface to the server?  (a la QMP)

But I wonder if you have any better ideas?

Rich.

-- 
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