While I'm thinking and waiting for suggestions on how to improve my file-system block cache algorithm, I've decided to apply some changes to the Raleigh File-System Format (source code is not published yet).
Following the ideas of Valerie Aurora of Repair-driven File System Design, I've decided to add for each block (B*Tree and Data blocks) an head that contains a Magic Number and a CRC Sum of the block. In this way you can easily identify what kind of block you've peeked without scanning all metadata. Another step is to add a back reference (or back pointer) to the data block, in this way you can easily jump back to it's the extent block (and obviously to its OID) so you can easily understand what is the Object owner of this block and you can easily swap two blocks reading at most 4 blocks (2 Data and 2 Extends).
Another idea stolen from Valerie is to double the metadata blocks with a COW-like approach, as explained in this paper "Double the Metadata, Double the Fun: A COW-like Approach to FS Consistency", really useful for personal file-systems but maybe less in a distributed file-system. I'm working on it adding only as an mkfs option.
When the source Code will be online? I don't know.. I've less time to work on it. Maybe at the end of this year I'll publish the File-System and the Distributed System (explained some posts ago).