X Notes on why eagier invalidation was rejected

wcfs/wcfs.go

@@ -288,7 +288,12 @@ package main
 //	FIXME no - we can build it but not in full - since we consider only zobj in live cache.
 //	FIXME and even if we consider all δ'ed zobj, building complete set of
 //	      file.δtail requires to first do complete scan of file.blktab
-//	      which is prohibitively expensive. XXX -> no, we'll do the scan
+//	      which is prohibitively expensive.
+//	XXX -> we'll do the scan, but only Trees _without_ Buckets. This
+//	       makes the scan practical until 1PB while helping to build
+//	       initial tracking set for δFtail.
+//	       Eager invalidation would require full scan - Trees _and_
+//	       Buckets, which makes it prohibitively expensive - see (+).
 //
 //    4.4) for all file/blk to invalidate we do:
 //