Re: [PATCH 0/4] btrfs: Make balance cancelling response faster

David Sterba
2019-12-04T16:40:02Z

On Tue, Dec 03, 2019 at 02:42:50PM +0800, Qu Wenruo wrote:
> [PROBLEM]
> There are quite some users reporting that 'btrfs balance cancel' slow to
> cancel current running balance, or even doesn't work for certain dead
> balance loop.
>
> With the following script showing how long it takes to fully stop a
> balance:
> #!/bin/bash
> dev=/dev/test/test
> mnt=/mnt/btrfs
>
> umount $mnt &> /dev/null
> umount $dev &> /dev/null
>
> mkfs.btrfs -f $dev
> mount $dev -o nospace_cache $mnt
>
> dd if=/dev/zero bs=1M of=$mnt/large &
> dd_pid=$!
>
> sleep 3
> kill -KILL $dd_pid
> sync
>
> btrfs balance start --bg --full $mnt &
> sleep 1
>
> echo "cancel request" >> /dev/kmsg
> time btrfs balance cancel $mnt
> umount $mnt
>
> It takes around 7~10s to cancel the running balance in my test
> environment.
>
> [CAUSE]
> Btrfs uses btrfs_fs_info::balance_cancel_req to record how many cancel
> request are queued.
> However that cancelling request is only checked after relocating a block
> group.

Yes that's the reason why it takes so long to cancel. Adding more
cancellation points is fine, but I don't know what exactly happens when
the block group relocation is not finished. There's code to merge the
reloc inode and commit that, but that's only a high-level view of the
thing.