the first 4 bytes of the relay log. Indeed comments in mysys/mf_iocache.c 
say we must always use my_b_append for such a cache.
This *could* avoid a very rare assertion failure which is: 
030524 19:32:38  Slave SQL thread initialized, starting replication in log 'FIRST' at position 0, relay log '/
users/gbichot/4.1.1/mysql-test/var/log/slave-relay-bin.000001' position: 4
030524 19:32:38  next log '/users/gbichot/4.1.1/mysql-test/var/log/slave-relay-bin.000002' is currently active
mysqld: mf_iocache.c:701: _my_b_seq_read: Assertion `pos_in_file == info->end_of_file' failed.
and which seemed to happen always when the SQL thread and/or the I/O thread
were at position 4 in a relay log.