[Gnu-arch-users] back-building pfs updates

[Aaron Bentley]

Merging back-building will have two impacts:
- we'll want better revision-selection heuristics
- most operations will no longer will need to be serial

Better heuristics
=================
We need to know how long a download will take.  Currently we can only
count revisions.  In order to estimate download time, we need to know
file size.  pfs does not provide this, but most supported protocols can.

This is another area in which http blows.  Can we add file size data to
the listing files?  Should we add other file data?

Parallel operations
===================
These operations could profitably be implemented as parallel operations:

- finding cacherevs
- calculating total download cost of a patch list
- retrieving a patch list
- finding the continuations of a list of package-versions

This would suggest making parallel versions of
file_exists
get_file
file_size

One way of doing this would be to "prime" the pfs.  Priming would mean
"I will probably want to do this, so be ready when I do".  The pfs could
then start doing the operations into buffers.  When the actual requests
came in, they would be satisfied from the buffer, blocking as necessary.

pfses that didn't support priming would simply treat priming commands as
noops.
Priming could be a mode, but considering the bad effects of forgetting
to turn the mode off (all operations are noops!), I think it would be
better to implement it as an alternate function or function parameter.

What do you make of all this?

Aaron

-- 
Aaron Bentley
Director of Technology
PanoMetrics, Inc.