For OSDs like ZFS to perform optimally it's import that they be allowed to manage their own cache. This maximizes the likelyhood that the ARC will prefetch and cache the right buffers. In the existing ZFS OSD code a cached LU object pins buffers in the ARC preventing them from being dropped. As the LU cache grows it can consume the entire ARC preventing buffers for other objects, such as the OIs, from being cached and severely impacting the performance for FID lookups.
The proposed patch addresses this by limiting the size of the lu_cache but alternate approaches are welcome. We are carrying this patch in LLNLs tree and it does help considerably.