Merge branch 't2' into t

* t2:
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wcfs/wcfs.go

@@ -308,7 +308,7 @@ package main
 //    4.4) processing ZODB invalidations and serving file reads (see 7) are
 //      organized to be mutually exclusive.
 //
-//    5.5) similarly, processing ZODB invalidations and setting up watches (see
+//    4.5) similarly, processing ZODB invalidations and setting up watches (see
 //      7.2) are organized to be mutually exclusive.
 //
 // 5) after OS file cache was invalidated, we resync zhead to new database
@@ -332,11 +332,16 @@ package main
 //
 //    min(rev) in δFtail is min(@at) at which head/bigfile/file is currently watched (see below).
 //
-//    to support initial openings with @at being slightly in the past, we also
+//    To support initial openings with @at being slightly in the past, we also
 //    make sure that min(rev) is enough to cover last 1 minute of history
 //    from head/at.
 //
-//    See ΔFtail documentation in internal/zdata/δftail.go for more details.
+//    Scalability of δFtail plays important role in scalability of WCFS because
+//    δFtail, besides other places, is queried and potentially rebuilt at every
+//    FUSE read request (see 7 below).
+//
+//    See documentation in internal/zdata/δftail.go for more details on ΔFtail
+//    and its scalability properties.
 //
 // 7) when we receive a FUSE read(#blk) request to a head/bigfile/file, we process it as follows:
 //