docs/dirnodes.txt: add notes on dirnode sizes

docs/dirnodes.txt

@@ -157,6 +157,52 @@ other users who have read-only access to 'foo' will be unable to decrypt its
 rwcap slot, this limits those users to read-only access to 'bar' as well,
 thus providing the transitive readonlyness that we desire.
 
+=== Dirnode sizes, mutable-file initial read sizes ===
+
+How big are dirnodes? When reading dirnode data out of mutable files, how
+large should our initial read be? If we guess exactly, we can read a dirnode
+in a single round-trip, and update one in two RTT. If we guess too high,
+we'll waste some amount of bandwidth. If we guess low, we need to make a
+second pass to get the data (or the encrypted privkey, for writes), which
+will cost us at least another RTT.
+
+Assuming child names are between 10 and 99 characters long, how long are the
+various pieces of a dirnode?
+
+ netstring(name) ~= 4+len(name)
+ chk-cap = 97 (for 4-char filesizes)
+ dir-rw-cap = 88
+ dir-ro-cap = 91
+ netstring(cap) = 4+len(cap)
+ encrypted(cap) = 16+cap+32
+ JSON({}) = 2
+ JSON({ctime=float,mtime=float}): 57
+ netstring(metadata) = 4+57 = 61
+
+so a CHK entry is:
+ 5+ 4+len(name) + 4+97 + 5+16+97+32 + 4+57
+And a 15-byte filename gives a 336-byte entry. When the entry points at a
+subdirectory instead of a file, the entry is a little bit smaller. So an
+empty directory uses 0 bytes, a directory with one child uses about 336
+bytes, a directory with two children uses about 672, etc.
+
+When the dirnode data is encoding using our default 3-of-10, that means we
+get 112ish bytes of data in each share per child.
+
+The pubkey, signature, and hashes form the first 935ish bytes of the
+container, then comes our data, then about 1216 bytes of encprivkey. So if we
+read the first:
+
+ 1kB: we get 65bytes of dirnode data : only empty directories
+ 1kiB: 89bytes of dirnode data : maybe one short-named subdir
+ 2kB: 1065bytes: about 9 entries
+ 3kB: 2065bytes: about 18 entries, or 7.5 entries plus the encprivkey
+ 4kB: 3065bytes: about 27 entries, or about 16.5 plus the encprivkey
+
+So we've written the code to do an initial read of 2kB from each share when
+we read the mutable file, which should give good performance (one RTT) for
+small directories.
+
 
 == Design Goals, redux ==