[Pvfs2-developers] md on md, dbpf attr cache, rmw, etc.

[Sam Lang]

Hi All,

I've been working on some changes recently to the DBPF  
implementation.  There some bits of the code that I've tried to  
cleanup, which I've tried to separate out in the attached patch.   
Please let me know what you think of these changes.  Here are the  
gory details:

* There are attributes on some objects which differ based on the type  
of object, but need to be stored outside the keyval space.   
Essentially, this is metadata on metadata.  For example, the metadata  
objects have the distribution size and the number of datafile  
handles, both of which vary from object to object, and we need to  
know those sizes before doing a keyval_read.  We store these size  
parameters in the dspace entry for the object.  Similarly, the  
dirdata object has a number of directory entries.  We were previously  
storing this value in a special "null" entry at the beginning of the  
sub-space for that handle's set of keyval entries.  This worked, but  
was always a bit of a hack.  The changes I've made gets rid of the  
keyval_handle_info, and stores all metadata-on-metadata in a union at  
the end of the dspace attributes.  This has the side affect of  
allowing us to get rid of the TROVE_ds_storedattr_s struct and just  
have one structure for storing and accessing dspace attributes.

* I changed the dbpf-attr-cache implementation to use our generic  
tcache (i.e. timeout cache -- in this case the timeout is  
infinity :-)).  There are now separate cache instances for dspace  
entries and keyval entries.  At present it still only caches keyval  
entries with certain keywords specified -- so directory entries don't  
get cached.

* Related to the above two bullets, I've added a per-handle mutex to  
the attribute cache.  This is needed to do read-modify-write on the  
the count of directory entries for each time a dirent is added or  
removed.  This should allow us to do fine grained locks on a per- 
handle basis in cases  like this where its needed.

Thanks,

-sam

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