Re: [PATCH v2 0/7] btrfs-progs: Support for BG_TREE feature

David Sterba
2019-10-22T12:23:39Z

On Tue, Oct 22, 2019 at 02:30:22PM +0800, Qu Wenruo wrote:
> BTW, there is one important compatibility problem related to all the BGI
> related features.
>
> Although I'm putting the BG_TREE feature as incompatible feature, but in
> theory, it should be RO compatible.

It could be RO compatible yes.

> As except extent/bg tree, we *should* read the fs without any problem.
>
> But the problem is, current btrfs mount (including btrfs-check) still
> need to go through the block group item search, even for permanent RO mount.
>
> This get my rescue mount option patchset to be involved.
> If we have such skip_bg feature earlier, we can completely afford to
> make all these potential features as RO compatible.
>
>
> Now my question is, should we put this feature still as incompatible
> feature?

In some way it would probably have to be incompat, either full or RO. As
unknown tree items are ignored, if the rest of the filesystem provides
enough information to access the data, then incompat RO sounds like the
best option. And that's probably independent of how exactly the new BGI
is done.