Re: [Xen-API] RRD library (was Re: Proposal to change committers for the XAPI Project)

[John Else <john.else@xxxxxxxxxx>]

On 02/06/14 15:48, Anil Madhavapeddy wrote:

> Thanks for the RRD library link -- I hadnt seen this before and it looks very 
> useful for Xen tracing in MirageOS! A few questions while reading through the 
> source:
> 
> - Rrd_io just exposes a single Resource_closed exception and no other values. 
>  That's perhaps easier to encode in the return value?

Heh, I'm not sure what I was thinking there...I agree :)

> 
> - Rrd_reader is functorized on its transport mechanism (for Gnt selection).  
> This should mean that the library can compile as a stub domain on Mirage, as 
> well as using the /dev/gnt* interfaces on Linux.  Is the current usecase to 
> run in dom0 or a separate RRD domain?

The most likely use case at the moment would be to run an rrdd plugin (writer) 
in a domU with rrdd (reader) running in dom0. However, I've tried to keep the 
library as generic as possible rather than building it for any particular use 
case - it works quite nicely for domU-domU communication. Check out the two 
programs under util/ if you want to have a play.

> 
> - Have you considered using vchan to let other tools capture the rrd stream 
> more easily?

I hadn't, but I think that could be made to work. At the moment the protocols 
expect the shared-memory resource to be represented as a cstruct so it would 
need a bit of tweaking to work with a socket-style interface instead.

I also wanted to keep the library free of dependencies to any particular 
handshake mechanism (e.g. xenstore), so I think the right interface would be to 
start with a ready-initialised vchan server and turn that into a reader (or 
conversely turn a vchan client into a writer).

John

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