[PATCH] bump up nr_to_write in xfs_vm_writepage

Eric Sandeen
2009-07-02T21:22:28Z

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@infradead.org>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Cc: Chris Mason <chris.mason@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.