diff --git a/fio.1 b/fio.1
index d372727..a6547ee 100644
--- a/fio.1
+++ b/fio.1
@@ -235,7 +235,9 @@
 Sets size to something really large and waits for ENOSPC (no space left on
 device) as the terminating condition. Only makes sense with sequential write.
 For a read workload, the mount point will be filled first then IO started on
-the result.
+the result. This option doesn't make sense if operating on a raw device node,
+since the size of that is already known by the file system. Additionally,
+writing beyond end-of-device will not return ENOSPC there.
 .TP
 .BI filesize \fR=\fPirange
 Individual file sizes. May be a range, in which case \fBfio\fR will select sizes