acache thrashing

[mtmoore]

There is an issue with acache thrashing / effectiveness.

Main issues

1. If optional attributes are part of a getattr request, and are not defined, when the client looks in the cache for the attribute it will consider the lack of optional attributes as a cache miss.

2. The size attribute is retrieved by itself and causes the other cached attributes to be invalidated (since any update to the acache invalidates the existing entry).

Possible changes

• no attribute masks used to confirm attributes in the cache
• modify the acache update behavior to merge existing entries and new entries
• remove the delineation between static and non-static attributes