diff --git a/fio.1 b/fio.1
index aa7b9d6..32993b6 100644
--- a/fio.1
+++ b/fio.1
@@ -544,7 +544,9 @@
 .BI pre_read \fR=\fPbool
 If this is given, files will be pre-read into memory before starting the given
 IO operation. This will also clear the \fR \fBinvalidate\fR flag, since it is
-pointless to pre-read and then drop the cache.
+pointless to pre-read 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.
 .TP
 .BI unlink \fR=\fPbool
 Unlink job files when done.  Default: false.