[Ocfs2-devel] [PATCH 1/2] ocfs2: Flush drive's caches on fdatasync

[Jan Kara <jack@suse.cz>]

On Fri 30-07-10 10:07:07, Tao Ma wrote:
> Hi Jan,
> 
> On 07/29/2010 08:00 PM, Jan Kara wrote:
> >We have to issue a cache flush during fdatasync even if inode doesn't have
> >I_DIRTY_DATASYNC set because we still have to get written *data* to disk to
> >observe fdatasync() guarantees.
> I am fine with the patch from the code's perspective.
> 
> But I just noticed the discussion in fsdevel with the subject
> "relaxed barrier semantics", so with barrier there will be a massive
> slowdowns according to Christoph. And as ocfs2 is mainly used with
> some SAN, I guess in most cases the storage will have a battery
> backed cache, so we may not need this?
  Yes, barriers are rather heavy operation and battery backed SANs don't
need them. In this regard, you are right my patch forgot to check the
OCFS2_MOUNT_BARRIER option. But when the option is set, we must send the
barrier. Otherwise we risk user's data on crash... Below is a fixed version
of the patch.

									Honza
---
From 61c46d394bdc9915554b499f7c3154bde13f3ee5 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Thu, 29 Jul 2010 13:32:19 +0200
Subject: [PATCH 1/2] ocfs2: Flush drive's caches on fdatasync

We have to issue a cache flush during fdatasync even if inode doesn't have
I_DIRTY_DATASYNC set because we still have to get written *data* to disk to
observe fdatasync() guarantees.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ocfs2/file.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
index 2b10b36..3ed8efd 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -36,6 +36,7 @@
 #include <linux/writeback.h>
 #include <linux/falloc.h>
 #include <linux/quotaops.h>
+#include <linux/blkdev.h>
 
 #define MLOG_MASK_PREFIX ML_INODE
 #include <cluster/masklog.h>
@@ -190,8 +191,16 @@ static int ocfs2_sync_file(struct file *file, int datasync)
 	if (err)
 		goto bail;
 
-	if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
+	if (datasync && !(inode->i_state & I_DIRTY_DATASYNC)) {
+		/*
+		 * We still have to flush drive's caches to get data to the
+		 * platter
+		 */
+		if (osb->s_mount_opt & OCFS2_MOUNT_BARRIER)
+			blkdev_issue_flush(inode->i_sb->s_bdev, GFP_KERNEL,
+					   NULL, BLKDEV_IFL_WAIT);
 		goto bail;
+	}
 
 	journal = osb->journal->j_journal;
 	err = jbd2_journal_force_commit(journal);
-- 
1.6.4.2