Add support for bs_is_seq_rand

This option will switch the meaning of the read,write part of any
blocksize setting to mean sequential,random instead.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/HOWTO b/HOWTO
index 8768b99..2335a07 100644
--- a/HOWTO
+++ b/HOWTO
@@ -505,6 +505,11 @@
 		may be used as a block range. This typically wont work with
 		direct IO, as that normally requires sector alignment.
 
+bs_is_seq_rand	If this option is set, fio will use the normal read,write
+		blocksize settings as sequential,random instead. Any random
+		read or write will use the WRITE blocksize settings, and any
+		sequential read or write will use the READ blocksize setting.
+
 zero_buffers	If this option is given, fio will init the IO buffers to
 		all zeroes. The default is to fill them with random data.