Re: Downgrading kernel 4.9 to 4.4 with space_cache=v2 enabled?

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-02-23 19:54, Qu Wenruo wrote:
>
>
> At 02/23/2017 06:51 PM, Christian Theune wrote:
>> Hi,
>>
>> not sure whether it’s possible, but we tried space_cache=v2 and
>> obviously after working fine in staging it broke in production. Or
>> rather: we upgraded from 4.4 to 4.9 and enabled the space_cache. Our
>> production volume is around 50TiB usable (underlying HW Raid 6).
>>
>> The machine crashes silently every 15 hours or so and takes _ages_ to
>> reboot. It current is stuck trying to mount the local filesystems and
>> I guess btrfs is doing something, but I don’t have shell access yet.
>>
>> I’m wondering whether we can downgrade by booting back into 4.4 or
>> will this break things even further? (We’ve had some unpleasant
>> surprises with FS’ in the last months, so I thought I’d rather ask.)
>
> You could use "btrfs check --clear-space-cache" to completely cleanup
> space cache. Both v1(free space cache, file based) or v2(free space
> tree) are supported.
>
> And specially for v2 space cache (space cache tree), "btrfs check
> --clear-space-cache" will also clear the ro_compat flag, so older kernel
> should mount the fs without problem.
That's really good to know.  I hadn't remembered that using mount 
options didn't clear the flag.