[BUG?] bcachefs: keep writing to device when there is no high-level I/O activity.

[David Wang <00107082@163.com>]

Hi,

I was using two partitions on same nvme device to compare filesystem performance,
and I consistantly observed a strange behavior:

After 10 minutes fio test with bcachefs on one partition, performance degrade
significantly for other filesystems on other partition (same device).

	ext4  150M/s --> 143M/s
	xfs   150M/s --> 134M/s
	btrfs 127M/s --> 108M/s

Several round tests show the same pattern that bcachefs seems occupy some device resource
even when there is no high-level I/O.

I monitor /proc/diskstats, and it confirmed that bcachefs do keep writing the device.
Following is the time serial samples for "writes_completed" on my bcachefs partition:

writes_completed @timestamp
	       0 @1724748233.712
	       4 @1724748248.712    <--- mkfs
	       4 @1724748263.712
	      65 @1724748278.712
	   25350 @1724748293.712
	   63839 @1724748308.712    <--- fio started
  	  352228 @1724748323.712
	  621350 @1724748338.712
	  903487 @1724748353.712
        ...
	12790311 @1724748863.712
	13100041 @1724748878.712
	13419642 @1724748893.712
	13701685 @1724748908.712    <--- fio done (10minutes)
	13701769 @1724748923.712    <--- from here, average 5~7writes/second for 2000 seconds
	13701852 @1724748938.712
	13701953 @1724748953.712
	13702032 @1724748968.712
	13702133 @1724748983.712
	13702213 @1724748998.712
	13702265 @1724749013.712
	13702357 @1724749028.712
        ...
	13712984 @1724750858.712
	13713076 @1724750873.712
	13713196 @1724750888.712
	13713299 @1724750903.712
	13713386 @1724750918.712
	13713463 @1724750933.712
	13713501 @1724750948.712   <--- writes stopped here
	13713501 @1724750963.712
	13713501 @1724750978.712
	...

Is this behavior expected? 

My test script:
	set -e
	for fsa in "btrfs" "ext4" "bcachefs" "xfs"
	do
		if [ $fsa == 'ext4' ]; then
			mkfs -t ext4 -F /dev/nvme0n1p1
		else
			mkfs -t $fsa -f /dev/nvme0n1p1
		fi
		mount -t $fsa /dev/nvme0n1p1 /disk02/dir1
		for fsb in "ext4" "bcachefs" "xfs" "btrfs"
		do
			if [ $fsb == 'ext4' ]; then
				mkfs -t ext4 -F /dev/nvme0n1p2
			else
				mkfs -t $fsb -f /dev/nvme0n1p2
			fi
			mount -t $fsb /dev/nvme0n1p2 /disk02/dir2

			cd /disk02/dir1 && fio --randrepeat=1 --ioengine=libaio --direct=1 --name=test  --bs=4k --iodepth=64 --size=1G --readwrite=randrw  --runtime=600 --numjobs=8 --time_based=1 --output=/disk02/fio.${fsa}.${fsb}.0
			sleep 30
			cd /disk02/dir2 && fio --randrepeat=1 --ioengine=libaio --direct=1 --name=test  --bs=4k --iodepth=64 --size=1G --readwrite=randrw  --runtime=600 --numjobs=8 --time_based=1 --output=/disk02/fio.${fsa}.${fsb}.1
			sleep 30
			cd /disk02
			umount /disk02/dir2
		done
		umount /disk02/dir1
	done

And here is a report for one round of test matrix:
+----------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
|   R|W    |             ext4            |           bcachefs          |             xfs             |            btrfs            |
+----------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
|   ext4   |    [ext4]147MB/s|147MB/s    |    [ext4]146MB/s|146MB/s    |    [ext4]150MB/s|150MB/s    |    [ext4]149MB/s|149MB/s    |
|          |    [ext4]146MB/s|146MB/s    | [bcachefs]72.2MB/s|72.2MB/s |     [xfs]149MB/s|149MB/s    |    [btrfs]132MB/s|132MB/s   |
| bcachefs | [bcachefs]71.9MB/s|71.9MB/s | [bcachefs]65.1MB/s|65.1MB/s | [bcachefs]69.6MB/s|69.6MB/s | [bcachefs]65.8MB/s|65.8MB/s |
|          |    [ext4]143MB/s|143MB/s    | [bcachefs]71.5MB/s|71.5MB/s |     [xfs]134MB/s|133MB/s    |    [btrfs]108MB/s|108MB/s   |
|   xfs    |     [xfs]148MB/s|148MB/s    |     [xfs]147MB/s|147MB/s    |     [xfs]152MB/s|152MB/s    |     [xfs]151MB/s|151MB/s    |
|          |    [ext4]147MB/s|147MB/s    | [bcachefs]71.3MB/s|71.3MB/s |     [xfs]148MB/s|148MB/s    |    [btrfs]127MB/s|127MB/s   |
|  btrfs   |    [btrfs]132MB/s|132MB/s   |    [btrfs]112MB/s|111MB/s   |    [btrfs]110MB/s|110MB/s   |    [btrfs]110MB/s|110MB/s   |
|          |    [ext4]147MB/s|146MB/s    | [bcachefs]69.7MB/s|69.7MB/s |     [xfs]146MB/s|146MB/s    |    [btrfs]125MB/s|125MB/s   |
+----------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
(The rows are for the FS on the first partition, and the cols are on the second partition)

The version of bcachefs-tools on my system is 1.9.1.
(The impact is worse, ext4 dropped to 80M/s, when I was using bcachefs-tools from debian repos which is too *old*,
and known to cause bcachefs problems. And that is the reason that I do this kind of test.)


Thanks
David