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[PATCH 07/10] repair: BMBT prefetch needs to be CRC aware

To: xfs@xxxxxxxxxxx
Subject: [PATCH 07/10] repair: BMBT prefetch needs to be CRC aware
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 27 Feb 2014 20:51:12 +1100
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1393494675-30194-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1393494675-30194-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

I'd been trying to track down a behavioural difference between
non-crc and crc enabled filesystems that was resulting non-crc
filesystem executing prefetch almost 3x faster than CRC filesystems.
After amny ratholes, I finally stumbled on the fact that btree
format directories are not being prefetched due to a missing magic
number check, and it's rejecting all XFS_BMAP_CRC_MAGIC format BMBT
buffers. This makes prefetch on CRC enabled filesystems behave the
same as for non-CRC filesystems.

The difference a single line of code can make on a 50 million inode
filesystem with a single threaded prefetch enabled run is pretty
amazing. It goes from 3,000 iops @ 50MB/s to 2,000 IOPS @ 800MB/s
and the cache hit rate goes from 3% to 49%. The runtime difference:

Unpatched:

Phase           Start           End             Duration
Phase 1:        02/21 18:34:12  02/21 18:34:12
Phase 2:        02/21 18:34:12  02/21 18:34:15  3 seconds
Phase 3:        02/21 18:34:15  02/21 18:40:09  5 minutes, 54 seconds
Phase 4:        02/21 18:40:09  02/21 18:46:36  6 minutes, 27 seconds
Phase 5:        02/21 18:46:36  02/21 18:46:37  1 second
Phase 6:        02/21 18:46:37  02/21 18:52:51  6 minutes, 14 seconds
Phase 7:        02/21 18:52:51  02/21 18:52:52  1 second

Total run time: 18 minutes, 40 seconds

Patched:

Phase           Start           End             Duration
Phase 1:        02/21 19:58:23  02/21 19:58:23
Phase 2:        02/21 19:58:23  02/21 19:58:27  4 seconds
Phase 3:        02/21 19:58:27  02/21 19:59:20  53 seconds
Phase 4:        02/21 19:59:20  02/21 20:00:07  47 seconds
Phase 5:        02/21 20:00:07  02/21 20:00:08  1 second
Phase 6:        02/21 20:00:08  02/21 20:00:50  42 seconds
Phase 7:        02/21 20:00:50  02/21 20:00:50

Total run time: 2 minutes, 27 seconds

Is no less impressive. Shame it's just a regression fix. ;)

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
---
 repair/prefetch.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/repair/prefetch.c b/repair/prefetch.c
index f4f3d71..946e822 100644
--- a/repair/prefetch.c
+++ b/repair/prefetch.c
@@ -298,7 +298,8 @@ pf_scanfunc_bmap(
        /*
         * do some validation on the block contents
         */
-       if ((be32_to_cpu(block->bb_magic) != XFS_BMAP_MAGIC) ||
+       if ((block->bb_magic != cpu_to_be32(XFS_BMAP_MAGIC) &&
+            block->bb_magic != cpu_to_be32(XFS_BMAP_CRC_MAGIC)) ||
                        (be16_to_cpu(block->bb_level) != level))
                return 0;
 
-- 
1.8.4.rc3

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