[Maas-devel] Dynamic TFTP server

[Jeroen Vermeulen]

On 2012-06-28 00:04, Gavin Panella wrote:
> I've worked a bit on a dynamic TFTP server. You can see my progress in
> lp:~allenap/maas/dynamic-tftp. This can field a TFTP request and
> dynamically render a PXE configuration file, falling back to the
> filesystem for non-matching paths. It's only rendering a stub right
> now, but next I'll hook it up to the existing PXEConfig stuff. I think
> that'll be quite easy.

Thanks for doing this.  I'd be quite interested to know how well the 
base implementation that you used holds up under stress — barring any 
problems there, doing this now does look simpler than continuing to 
manage the PXE downloads purely through the filesystem.


As we discussed, and you asked me to explain here in more detail, there 
is a loosely related issue of the static files (pre-bootloader, kernel, 
initrd) and how to keep them consistent.  A booting node ought to 
receive an initrd that matches its kernel, and since the TFTP server 
keeps no sessions, neither of those files should be replaced while being 
downloaded.

We can't stop nodes from booting at inopportune times, so I think we'll 
have to go with a versioning scheme — maintain multiple versions of 
these files, so nodes that are booting from older versions can continue 
using those even when newer versions become available during the boot. 
We'd garbage-collect versions that are no longer needed.

With a dynamic TFTP server we have two ways to do versioning: explicit 
or transparent.

  * “Explicit”: keep a version number visible somewhere in its TFTP 
path.  So for instance, you could direct a node to load kernel 
/maas/amd64/precise/install-123/linux.

  * “Transparent”: hide the version numbers from the TFTP paths, but 
remember that a node is booting off a particular version and 
transparently serve it that version.  The path would still be like 
/maas/amd64/precise/install/linux as it is today.

I think the explicit approach is probably best, because it naturally 
keeps the choice of version as node-side state.  On the flip side, it 
will complicate our TFTP path handling a bit, and we'll have to keep 
track of the current version somewhere.


Jeroen