This patch implements O_DIRECT about 80% of the way. It takes a patchset

Tor created a while ago, removes the raw I/O piece (that has cache coherency
problems), and adds a buffer cache / VM freeing piece.

Essentially this patch causes O_DIRECT I/O to not be left in the cache, but
does not prevent it from going through the cache, hence the 80%.  For
the last 20% we need a method by which the I/O can be issued directly to
buffer supplied by the user process and bypass the buffer cache entirely,
but still maintain cache coherency.

I also have the code working under -stable but the changes made to sys/file.h
may not be MFCable, so an MFC is not on the table yet.

Submitted by:	tegge, dillon

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/libc/sys/open.2 b/lib/libc/sys/open.2
index 769ac27..1a29a17 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.