[PATCH] bump up nr_to_write in xfs_vm_writepage

[Eric Sandeen]

Talking w/ someone who had a raid6 of 15 drives on an areca
controller, he wondered why he could only get 300MB/s or so
out of a streaming buffered write to xfs like so:

dd if=/dev/zero of=/mnt/storage/10gbfile bs=128k count=81920
10737418240 bytes (11 GB) copied, 34.294 s, 313 MB/s

when the same write directly to the device was going closer
to 700MB/s...

With the following change things get moving again for xfs:

dd if=/dev/zero of=/mnt/storage/10gbfile bs=128k count=81920
10737418240 bytes (11 GB) copied, 16.2938 s, 659 MB/s

Chris had sent out something similar at Christoph's suggestion,
and Christoph reminded me of it, and I tested it a variant of
it, and it seems to help shockingly well.

Feels like a bandaid though; thoughts?  Other tests to do?

Thanks,
-Eric

Signed-off-by: Christoph Hellwig <hch at infradead.org>
Signed-off-by: Eric Sandeen <sandeen at sandeen.net>
Cc: Chris Mason <chris.mason at oracle.com>
---

Index: linux-2.6/fs/xfs/linux-2.6/xfs_aops.c
===================================================================
--- linux-2.6.orig/fs/xfs/linux-2.6/xfs_aops.c
+++ linux-2.6/fs/xfs/linux-2.6/xfs_aops.c
@@ -1268,6 +1268,13 @@ xfs_vm_writepage(
 	if (!page_has_buffers(page))
 		create_empty_buffers(page, 1 << inode->i_blkbits, 0);
 
+
+	/*
+	 *  VM calculation for nr_to_write seems off.  Bump it way
+	 *  up, this gets simple streaming writes zippy again.
+	 */
+	wbc->nr_to_write *= 4;
+
 	/*
 	 * Convert delayed allocate, unwritten or unmapped space
 	 * to real space and flush out to disk.