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[PATCH v3 2/2] tools/xenstore: set open file descriptor limit for xenstored

Juergen Gross writes ("[PATCH v3 2/2] tools/xenstore: set open file descriptor 
limit for xenstored"):
> Add a configuration item for the maximum number of domains xenstored
> should support and set the limit of open file descriptors accordingly.
> For HVM domains there are up to 5 socket connections per domain (2 by
> the xl daemon process, and 3 by qemu). So set the ulimit for xenstored
> to 5 * XENSTORED_MAX_DOMAINS + 100 (the "+ 100" is for some headroom,
> like logging, event channel device, etc.).
> +## Type: integer
> +## Default: 32768
> +#
> +# Select maximum number of domains supported by xenstored.
> +# Only evaluated if XENSTORETYPE is "daemon".

I approve of doing something about the fd limit.  I have some qualms
about the documentation.

The documentation doesn't say what happens if this limit is exceeded.
Also the default of 32758 suggests that we actually support that many
domains.  I don't think we do...

I didn't find anything in SUPPORT.md about how many guests we support
but I wouldn't want this setting here to imply full support for 32768

If you don't want to tackle this can of works, maybe add this:

  # This just controls some resource limits for xenstored; if the
  # limit is exceeded, xenstored will stop being able to function
  # properly for additional guests.  The default value is so large
  # that it won't be exceeded in a supported configuration, but
  # should not be taken to mean that the whole Xen system is
  # guaranteed to work properly with that many guests.

Julien, did you ask for this to be made configurable ?  Having written
the text above, I wonder if it wouldn't just be better to
unconditionally set it to "unlimited" rather than offering footgun
dressed up like a tuneable...

If xenstored does go mad or leadk lots of fds, things are basically
stuffed in anycase.  Having its syscalls fail with EMFILE is not
really going to help.




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