doc: add ftruncate engine documentation and example jobfile

Signed-off-by: Sitsofe Wheeler <[email protected]>

HOWTO

@@ -1675,6 +1675,11 @@ I/O engine
 			DDIR_TRIM
 				does fallocate(,mode = FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE).
 
+		**ftruncate**
+			I/O engine that sends :manpage:`ftruncate(2)` operations in response
+			to write (DDIR_WRITE) events. Each ftruncate issued sets the file's
+			size to the current block offset. Block size is ignored.
+
 		**e4defrag**
 			I/O engine that does regular EXT4_IOC_MOVE_EXT ioctls to simulate
 			defragment activity in request to DDIR_WRITE event.

examples/ftruncate.fio

@@ -0,0 +1,27 @@
+# Example ftruncate engine jobs
+
+[global]
+ioengine=ftruncate
+directory=/scratch
+size=102404k ; 100Mb+4k
+stonewall
+filename=truncate
+runtime=10s
+time_based
+direct=1
+#
+# bs option is stub here. Truncation is performed on the current block offset.
+# blocksize value is ignored
+bs=4k
+
+# truncate the file to 4Kbytes then repeatedly grow the file back to just over
+# its original size using subsequent truncates
+[grow-truncate]
+rw=write
+
+# Repeatedly change a file to a random size between 0Kbytes and 100Mb
+# using truncates
+[rand-truncate]
+rw=randwrite
+norandommap
+