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, 5 insertions, 0 deletions

diff --git a/lib/libc/sys/fcntl.2 b/lib/libc/sys/fcntl.2
index b2911e5..479e097 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