[Pvfs2-developers] TroveMethod configuration parameter

[Phil Carns]

> The main issue here is that the trove initialize doesn't know which  
> method to use, so there needs to be a global default.  Each  collection 
> also gets its own method as you've noticed, so you can  specify that on 
> a per collection basis.

Ok- that makes sense.  Thanks for the confirmation.

>> The pvfs2-genconfig utility places TroveMethod option in the  
>> <Defaults> section.  Is this intentional?  For example, if someone  
>> edited a configuration file created by genconfig, they might try to  
>> change the value of TroveMethod from "dbpf" to "alt-aio".  However,  
>> it doesn't look like this would have any real impact.  It would  
>> change the parameter to trove's initialize() function, but the  
>> collection_lookup() would still default to the dbpf method.
> 
> Hmm.. that's true.  The config format and the trove interfaces don't  
> match up real well.  Ideally, there wouldn't be a TroveMethod in the  
> <Defaults> section at all, and the trove initialize would work on a  
> per-collection basis, but right now we store collection info in both  
> the config file and inside the collections database.  To be able to  
> open and read from the collections database, we need a trove method.
> 
> Maybe it makes sense to use the default from the <Defaults> section  if 
> one isn't specified for that collection?  I'm not crazy about this  
> solution but it seems like a reasonable alternative.

That seems a little more intuitive.  Most people editing the config file 
would probably expect behavior like that (not knowing what the trove 
stack looks like).  Here are a couple of other ideas to throw out there:

B) split this into two keywords (hopefully with better names than the 
examples below):

- TroveCollectionMethod: valid _only_ in the StorageHints section
- TroveStorageSpaceMethod: valid _only_ in the Defaults section

C) keep the existing scheme, but just with two tweaks:

- clarify the comments/documentation in server-config.c to indicate that 
the parameter means something a little different depending on where it 
is used
- change pvfs2-genconfig to emit the "TroveMethod dbpf" line in the 
StorageHints section rather than the Defaults section.  That way someone 
  who comes along later and edits the file would tend to change it in 
the place that has an impact on server performance.

For what its worth, I am going option C) here for the time being at our 
site.

-Phil