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diff --git a/HOWTO b/HOWTO
index 1e1f4097..996da23a 100644
--- a/HOWTO
+++ b/HOWTO
@@ -224,7 +224,9 @@ bs=siint	The block size used for the io units. Defaults to 4k. Values
 		after a comma, it will apply to writes only. In other words,
 		the format is either bs=read_and_write or bs=read,write.
 		bs=4k,8k will thus use 4k blocks for reads, and 8k blocks
-		for writes.
+		for writes. If you only wish to set the write size, you
+		can do so by passing an empty read size - bs=,8k will set
+		8k for writes and leave the read default value.
 
 bsrange=irange	Instead of giving a single block size, specify a range
 		and fio will mix the issued io block sizes. The issued
@@ -262,6 +264,10 @@ ioengine=str	Defines how the job issues io to the file. The following
 				we use read(2) and write(2) for asynchronous
 				io.
 
+			null	Doesn't transfer any data, just pretends
+				to. This is mainly used to exercise fio
+				itself and for debugging/testing purposes.
+
 iodepth=int	This defines how many io units to keep in flight against
 		the file. The default is 1 for each file defined in this
 		job, can be overridden with a larger value for higher
@@ -351,11 +357,32 @@ mem=str		Fio can use various types of memory as the io unit buffer.
 			shm	Use shared memory as the buffers. Allocated
 				through shmget(2).
 
+			shmhuge	Same as shm, but use huge pages as backing.
+
 			mmap	Use anonymous memory maps as the buffers.
 				Allocated through mmap(2).
 
+			mmaphuge Use a memory mapped huge file as the buffer
+				backing. Append filename after mmaphuge, ala
+				mem=mmaphuge:/hugetlbfs/file
+
 		The area allocated is a function of the maximum allowed
-		bs size for the job, multiplied by the io depth given.
+		bs size for the job, multiplied by the io depth given. Note
+		that for shmhuge and mmaphuge to work, the system must have
+		free huge pages allocated. This can normally be checked
+		and set by reading/writing /proc/sys/vm/nr_hugepages on a
+		Linux system. Fio assumes a huge page is 4MiB in size. So
+		to calculate the number of huge pages you need for a given
+		job file, add up the io depth of all jobs (normally one unless
+		iodepth= is used) and multiply by the maximum bs set. Then
+		divide that number by the huge page size. You can see the
+		size of the huge pages in /proc/meminfo. If no huge pages
+		are allocated by having a non-zero number in nr_hugepages,
+		using mmaphuge or shmhuge will fail.
+
+		mmaphuge also needs to have hugetlbfs mounted and the file
+		location should point there. So if it's mounted in /huge,
+		you would use mem=mmaphuge:/huge/somefile.
 
 exitall		When one job finishes, terminate the rest. The default is
 		to wait for each job to finish, sometimes that is not the