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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2019/12/5 上午10:58, Zygo Blaxell wrote:
> 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.
>>
>> That behavior is far from optimal to provide a faster cancelling.
>>
>> [FIX]
>> This patchset will add more cancelling check points, to make cancelling
>> faster.
> 
> Nice!  I look forward to using this in the future!
> 
> Does this cover device delete/resize as well?

Shrink also takes use of balance, so I see no reason why it won't work
on such use cases.

>  I think there needs to be
> a check added for fatal signals for those to work, as they don't respond
> to balance cancel.

That's a good extra idea.

Since we have that wrapper, it would be easier to add in the future.

Thanks,
Qu

> 
>> And also, introduce a new error injection points to cover these newly
>> introduced and future check points.
>>
>> Qu Wenruo (4):
>>   btrfs: relocation: Introduce error injection points for cancelling
>>     balance
>>   btrfs: relocation: Check cancel request after each data page read
>>   btrfs: relocation: Check cancel request after each extent found
>>   btrfs: relocation: Work around dead relocation stage loop
>>
>>  fs/btrfs/ctree.h      |  1 +
>>  fs/btrfs/relocation.c | 23 +++++++++++++++++++++++
>>  fs/btrfs/volumes.c    |  2 +-
>>  3 files changed, 25 insertions(+), 1 deletion(-)
>>
>> -- 
>> 2.24.0
>>


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