Document that write_iolog is unsafe for concurrent jobs

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 7cb24dd8c12e764a7ff5c5c5c813efc775418311..7c943294ecfb838ec79ab1f6d558ab4390e2be32 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -549,7 +549,14 @@ ioengine=str       Defines how the job issues io to the file. The following
 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
-               concurrency.
+               concurrency. Note that increasing iodepth beyond 1 will not
+               affect synchronous ioengines (except for small degress when
+               verify_async is in use). Even async engines my impose OS
+               restrictions causing the desired depth not to be achieved.
+               This may happen on Linux when using libaio and not setting
+               direct=1, since buffered IO is not async on that OS. Keep an
+               eye on the IO depth distribution in the fio output to verify
+               that the achieved depth is as expected. Default: 1.
 
 iodepth_batch_submit=int
 iodepth_batch=int This defines how many pieces of IO to submit at once.
@@ -981,7 +988,8 @@ zoneskip=int        Skip the specified number of bytes when zonesize data has
                io on zones of a file.
 
 write_iolog=str        Write the issued io patterns to the specified file. See
-               read_iolog.
+               read_iolog.  Specify a separate file for each job, otherwise
+               the iologs will be interspersed and the file may be corrupt.
 
 read_iolog=str Open an iolog with the specified file name and replay the
                io patterns it contains. This can be used to store a