Re: fiemap is slow on btrfs on files with multiple extents

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Aug 04, 2022 at 07:30:52PM +0300, Pavel Tikhomirov wrote:
> I ran the below test on Fedora 36 (the test basically creates "very" sparse
> file, with 4k data followed by 4k hole again and again for the specified
> length and uses fiemap to count extents in this file) and face the problem
> that fiemap hangs for too long (for instance comparing to ext4 version).
> Fiemap with 32768 extents takes ~37264 us and with 65536 extents it takes
> ~34123954 us, which is x1000 times more when file only increased twice the
> size:
>

Ah that was helpful, thank you.  I think I've spotted the problem, please give
this a whirl to make sure we're seeing the same thing.  Thanks,

Josef
 
From 1133d5ebf952ebf334bc7be21a575b1f52eb71d4 Mon Sep 17 00:00:00 2001
Message-Id: <1133d5ebf952ebf334bc7be21a575b1f52eb71d4.1659638886.git.josef@toxicpanda.com>
From: Josef Bacik <josef@toxicpanda.com>
Date: Thu, 4 Aug 2022 14:45:53 -0400
Subject: [PATCH] btrfs: don't search entire range for delalloc with fiemap

For the case where we have

[EXTENT1][HOLE][EXTENT2]

If we fiemap from [HOLE] we will search to len (which could be -1) to
see if there's any delalloc extents in the range, however in the above
case btrfs_get_extent() returns a hole em for just the range of the
hole, as it will find EXTENT2, so all we need to do is search for
delalloc in the hole range, not the entire rest of the requested fiemap
range.

This fixes the extremely bad fiemap performance with very large sparse
files.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 8fc1e3b6e00c..b7ad8f7a7b53 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7095,7 +7095,7 @@ struct extent_map *btrfs_get_extent_fiemap(struct btrfs_inode *inode,
 		hole_em = em;
 
 	/* check to see if we've wrapped (len == -1 or similar) */
-	end = start + len;
+	end = em->start + em->len;
 	if (end < start)
 		end = (u64)-1;
 	else
-- 
2.36.1