doc: clarify the usage of rw_sequencer

Update man page clarifying the usage of rw_sequencer=sequential
Added few examples explaining the offset generation for rw_sequencer.

Fixes: https://github.com/axboe/fio/issues/1223

Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>

diff --git a/HOWTO.rst b/HOWTO.rst
index 0a48a453774b574773ef8ea31b63ccb5db1598e6..17caaf5d38f2972d5abe997b0ddb2e3a3649740f 100644 (file)
--- a/HOWTO.rst
+++ b/HOWTO.rst
@@ -1176,13 +1176,34 @@ I/O type
                        Generate the same offset.
 
        ``sequential`` is only useful for random I/O, where fio would normally
-       generate a new random offset for every I/O. If you append e.g. 8 to randread,
-       you would get a new random offset for every 8 I/Os. The result would be a
-       seek for only every 8 I/Os, instead of for every I/O. Use ``rw=randread:8``
-       to specify that. As sequential I/O is already sequential, setting
-       ``sequential`` for that would not result in any differences.  ``identical``
-       behaves in a similar fashion, except it sends the same offset 8 number of
-       times before generating a new offset.
+       generate a new random offset for every I/O. If you append e.g. 8 to
+       randread, i.e. ``rw=randread:8`` you would get a new random offset for
+       every 8 I/Os. The result would be a sequence of 8 sequential offsets
+       with a random starting point. However this behavior may change if a
+       sequential I/O reaches end of the file. As sequential I/O is already
+       sequential, setting ``sequential`` for that would not result in any
+       difference. ``identical`` behaves in a similar fashion, except it sends
+       the same offset 8 number of times before generating a new offset.
+
+       Example #1::
+
+               rw=randread:8
+               rw_sequencer=sequential
+               bs=4k
+
+       The generated sequence of offsets will look like this:
+       4k, 8k, 12k, 16k, 20k, 24k, 28k, 32k, 92k, 96k, 100k, 104k, 108k,
+       112k, 116k, 120k, 48k, 52k ...
+
+       Example #2::
+
+               rw=randread:8
+               rw_sequencer=identical
+               bs=4k
+
+       The generated sequence of offsets will look like this:
+       4k, 4k, 4k, 4k, 4k, 4k, 4k, 4k, 92k, 92k, 92k, 92k, 92k, 92k, 92k, 92k,
+       48k, 48k, 48k ...
 
 .. option:: unified_rw_reporting=str
 

diff --git a/fio.1 b/fio.1
index eb87533fc7932e979373a927b8d935a00627012f..527b3d4605d261b75bd868280e59bd21f81cf0a1 100644 (file)
--- a/fio.1
+++ b/fio.1
@@ -952,12 +952,47 @@ Generate the same offset.
 .P
 \fBsequential\fR is only useful for random I/O, where fio would normally
 generate a new random offset for every I/O. If you append e.g. 8 to randread,
-you would get a new random offset for every 8 I/Os. The result would be a
-seek for only every 8 I/Os, instead of for every I/O. Use `rw=randread:8'
-to specify that. As sequential I/O is already sequential, setting
-\fBsequential\fR for that would not result in any differences. \fBidentical\fR
-behaves in a similar fashion, except it sends the same offset 8 number of
-times before generating a new offset.
+i.e. `rw=randread:8' you would get a new random offset for every 8 I/Os. The
+result would be a sequence of 8 sequential offsets with a random starting
+point.  However this behavior may change if a sequential I/O reaches end of the
+file. As sequential I/O is already sequential, setting \fBsequential\fR for
+that would not result in any difference. \fBidentical\fR behaves in a similar
+fashion, except it sends the same offset 8 number of times before generating a
+new offset.
+.P
+.P
+Example #1:
+.RS
+.P
+.PD 0
+rw=randread:8
+.P
+rw_sequencer=sequential
+.P
+bs=4k
+.PD
+.RE
+.P
+The generated sequence of offsets will look like this:
+4k, 8k, 12k, 16k, 20k, 24k, 28k, 32k, 92k, 96k, 100k, 104k, 108k, 112k, 116k,
+120k, 48k, 52k ...
+.P
+.P
+Example #2:
+.RS
+.P
+.PD 0
+rw=randread:8
+.P
+rw_sequencer=identical
+.P
+bs=4k
+.PD
+.RE
+.P
+The generated sequence of offsets will look like this:
+4k, 4k, 4k, 4k, 4k, 4k, 4k, 4k, 92k, 92k, 92k, 92k, 92k, 92k, 92k, 92k, 48k,
+48k, 48k ...
 .RE
 .TP
 .BI unified_rw_reporting \fR=\fPstr