RE: Xenstore quota and driver domains

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Jürgen 
> Groß
> Sent: 04 June 2020 06:03
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Xenstore quota and driver domains
> 
> 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).

Which quota is hit? Node or watch?

> 
> 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?

Even though 1 requires more code churn, I still think it is the better way to 
go.

  Paul

> 
> 
> Juergen