On Tue, Jun 11, 2019 at 11:53:15AM +0200, David Sterba wrote: > On Mon, Jun 10, 2019 at 02:48:06PM +0000, Hugo Mills wrote: > > On Mon, Jun 10, 2019 at 04:02:36PM +0200, David Sterba wrote: > > > On Mon, Jun 10, 2019 at 12:42:26PM +0000, Hugo Mills wrote: > > > > Hi, David, > > > > > > > > On Mon, Jun 10, 2019 at 02:29:40PM +0200, David Sterba wrote: > > > > > this patchset brings the RAID1 with 3 and 4 copies as a separate > > > > > feature as outlined in V1 > > > > > (https://lore.kernel.org/linux-btrfs/cover.1531503452.git.dste...@suse.com/). > > > > [...] > > > > > Compatibility > > > > > ~~~~~~~~~~~~~ > > > > > > > > > > The new block group types cost an incompatibility bit, so old kernel > > > > > will refuse to mount filesystem with RAID1C3 feature, ie. any chunk on > > > > > the filesystem with the new type. > > > > > > > > > > To upgrade existing filesystems use the balance filters eg. from RAID6 > > > > > > > > > > $ btrfs balance start -mconvert=raid1c3 /path > > > > [...] > > > > > > > > If I do: > > > > > > > > $ btrfs balance start -mprofiles=raid13c,convert=raid1 \ > > > > -dprofiles=raid13c,convert=raid6 /path > > > > > > > > will that clear the incompatibility bit? > > > > > > No the bit will stay, even though there are no chunks of the raid1c3 > > > type. Same for raid5/6. > > > > > > Dropping the bit would need an extra pass trough all chunks after > > > balance, which is feasible and I don't see usability surprises. That you > > > ask means that the current behaviour is probably opposite to what users > > > expect. > > > > We've had a couple of cases in the past where people have tried out > > a new feature on a new kernel, then turned it off again and not been > > able to go back to an earlier kernel. Particularly in this case, I can > > see people being surprised at the trapdoor. "I don't have any RAID13C > > on this filesystem: why can't I go back to 5.2?" > > Undoing the incompat bit is expensive in some cases, eg. for ZSTD this > would mean to scan all extents, but in case of the raid profiles it's > easy to check the list of space infos that are per-profile. > > So, my current idea is to use the sysfs interface. The /features > directory lists the files representing features and writing 1 to the > file followed by a sync would trigger the rescan and drop the bit > eventually. > > The meaning of the /sys/fs/btrfs/features/* is defined for 1, which > means 'can be set at runtime', so the ability to unset the feature would > be eg. 3, as a bitmask of possible actions (0b01 set, 0b10 unset). > > We do have infrastructure for changing the state in a safe manner even > from sysfs, which sets a bit somewhere and commit processes that. That's > why the sync is required, but I don't think that's harming usability t
Scratch that, there's much simpler way and would work as expected in the example. Ie. after removing last bg of the given type the incompat bit will be dropped.
