Re: [PATCH v2 0/3] btrfs: separate BLOCK_GROUP_TREE feature from extent-tree-v2

[David Sterba <dsterba@suse.cz>]

On Wed, Jul 27, 2022 at 05:47:25AM +0800, Qu Wenruo wrote:
> 
> 
> On 2022/7/27 01:59, David Sterba wrote:
> > On Wed, Jul 20, 2022 at 01:06:58PM +0800, Qu Wenruo wrote:
> >> Qu Wenruo (3):
> >>    btrfs: enhance unsupported compat RO flags handling
> >>    btrfs: don't save block group root into super block
> >>    btrfs: separate BLOCK_GROUP_TREE compat RO flag from EXTENT_TREE_V2
> >
> > It's short series and I don't see any new code to use the separate tree
> > for bg items, so it's on top of the extent tree v2, right?
> 
> Yes, it's based on extent tree v2 prepare code that is already in the
> mainline code.
> 
> >
> >  From the last time we were experimenting with the block group tree, I
> > was trying to avoid a new tree but there were problems. So, I think we
> > can go with the separate tree that you suggest. We have reports about
> > slow mount and people use large filesystems, so this is justified.
> >
> > Will it be possible to convert existing filesystem to use the bg tree?
> 
> Yes, that's completely planned as the old bg tree code, btrfs-progs
> convert tool will be provided (mostly in btrfstune).

Ok, good. I'm thinking if we should go for an online conversion too or
not, because on a many-TB filesystem it would possibly take a long time
but the benefit is not to unmount and do the conversion.

We could copy what the free space conversion does on remount, for bg
tree implemented as "set some flag via sysfs" and ten trigger remount
that does all th work. It should be less or comparable work to free
space tree conversion, it's basically copying the block group items to
the new tree and deleting from extent tree.