Xenstore quota and driver domains

[Jürgen Groß <jgross@xxxxxxxx>]

A recent report on xen-users surfaced a problem we have with driver
domains in medium sized or large configuration: the driver domain can
easily hit the default Xenstore quota (in the report it was a driver
domain for disks which hit the quota when 15 domUs were active at the
same time).

Setting the quota to a higher limit will work, of course, but this
enables "normal" domUs to use this quota, too, thus hogging lots of
Xenstore resources.

I believe the most sensible way to solve that would be to have a way
to set per-domain quota in Xenstore. As the original reporter already
raised concerns regarding rebooting the server for applying new global
quota, I believe new quota settings should be possible at runtime.

The question is how this interface should look like. Right now I could
think of two variants:

1. A new XS_SET_QUOTA and XS_GET_QUOTA wire command pair which can set
   and get the quota (both, default values for new domains and for any
   existing domain)

2. A new XS_CONTROL command for setting/getting quota (same scope as 1.)

Both variants have advantages and disadvantages:

1. Pros: - clear interface, easily usable
   Cons: - requires more code churn in tools (libxenstore, xl, libxl)
           for support in domain-config
         - any extension will again require similar churn

2. Pros: - easy extensible domain-config possible (via a single item,
           e.g. "xenstore-quota='watches=10 entries=2000'")
   Cons: - text parsing in Xenstore instead of xl/libxl
         - XS_CONTROL sub-options for quota will be ABI

Any thoughts?


Juergen