Don't allow pre_read on IO engines that cannot seek

[fio.git] / HOWTO

diff --git a/HOWTO b/HOWTO
index 3107d3a15e6b4488dcd8626264b791d078459d1e..708eca0037293e80cc681c17f20d6638dc5c4cbd 100644 (file)
--- a/HOWTO
+++ b/HOWTO
@@ -741,7 +741,10 @@ create_on_open=bool        Don't pre-setup the files for IO, just create open()
 pre_read=bool  If this is given, files will be pre-read into memory before
                starting the given IO operation. This will also clear
                the 'invalidate' flag, since it is pointless to pre-read
 pre_read=bool  If this is given, files will be pre-read into memory before
                starting the given IO operation. This will also clear
                the 'invalidate' flag, since it is pointless to pre-read

-               and then drop the cache.
+               and then drop the cache. This will only work for IO engines
+               that are seekable, since they allow you to read the same data
+               multiple times. Thus it will not work on eg network or splice
+               IO.

 
 unlink=bool    Unlink the job files when done. Not the default, as repeated
                runs of that job would then waste time recreating the file
 
 unlink=bool    Unlink the job files when done. Not the default, as repeated
                runs of that job would then waste time recreating the file