2 files changed, 12 insertions, 0 deletions

diff --git a/lib/libc/sys/fcntl.2 b/lib/libc/sys/fcntl.2
index b2911e584047..479e0973b0a9 100644
--- a/lib/libc/sys/fcntl.2
+++ b/lib/libc/sys/fcntl.2
@@ -147,6 +147,11 @@ corresponds to the
 .Dv O_APPEND
 flag of
 .Xr open 2 .
+.It Dv O_DIRECT
+Minimize or eliminate the cache effects of reading and writing.  The system
+will attempt to avoid caching the data you read or write.  If it cannot
+avoid caching the data, it will minimize the impact the data has on the cache.
+Use of this flag can drastically reduce performance if not used with care.
 .It Dv O_ASYNC
 Enable the
 .Dv SIGIO
diff --git a/lib/libc/sys/open.2 b/lib/libc/sys/open.2
index 769ac27f0e01..1a29a17d3690 100644
--- a/lib/libc/sys/open.2
+++ b/lib/libc/sys/open.2
@@ -83,6 +83,7 @@ O_TRUNC		truncate size to 0
 O_EXCL		error if create and file exists
 O_SHLOCK	atomically obtain a shared lock
 O_EXLOCK	atomically obtain an exclusive lock
+O_DIRECT	eliminate or reduce cache effects
 O_FSYNC		synchronous writes
 O_NOFOLLOW	do not follow symlinks
 .Ed
@@ -150,6 +151,12 @@ If creating a file with
 the request for the lock will never fail
 (provided that the underlying filesystem supports locking).
 .Pp
+.Dv O_DIRECT may be used to
+minimize or eliminate the cache effects of reading and writing.  The system
+will attempt to avoid caching the data you read or write.  If it cannot
+avoid caching the data, it will minimize the impact the data has on the cache.
+Use of this flag can drastically reduce performance if not used with care. 
+.Pp
 If successful,
 .Fn open
 returns a non-negative integer, termed a file descriptor.