Yep that's what I'm referring to here: 
https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)

Thanks for the prompt respone Qu!

So just to confirm, our scheduled cron jobs to scrub will still work 
with space_cache=v2?



On 14/7/21 4:05 pm, Qu Wenruo wrote:

On 2021/7/14 下午1:44, Chris Murphy wrote:

quoted

On Tue, Jul 13, 2021 at 10:59 PM Qu Wenruo [off-list ref] 
wrote:

quoted

On 2021/7/13 下午11:38, DanglingPointer wrote:

quoted

quoted

2. If we use space_cache=v2, is it indeed still the case that the
     "btrfs" command will NOT work with the filesystem?

Why would you think "btrfs" won't work on a btrfs?

Maybe this?

man 5 btrfs, space_cache includes:

  The btrfs(8) command currently only has read-only support for v2. A
read-write command may be run on a v2 filesystem by clearing the
cache, running the command, and then remounting with space_cache=v2.

Oh, that's only for offline tools writing into the fs, namingly "btrfs
check --repair" and "mkfs.btrfs -R"

And I believe that sentence is now out-of-date after btrfs-progs v4.19,
which pulls all the support for write time free space tree (v2 space 
cache).

I'll soon send out a patch to fix that.

Thanks,
Qu