On Wed, Oct 24, 2018 at 03:24:29PM -0400, Mikulas Patocka wrote: > > > On Wed, 24 Oct 2018, Paul Lawrence wrote: > > > Android has had the concept of A/B updates for since Android N, which means > > that if an update is unable to boot for any reason three times, we revert to > > the older system. However, if the failure occurs after the new system has > > started modifying userdata, we will be attempting to start an older system > > with a newer userdata, which is an unsupported state. Thus to make A/B able to > > fully deliver on its promise of safe updates, we need to be able to revert > > userdata in the event of a failure. > > > > For those cases where the file system on userdata supports > > snapshots/checkpoints, we should clearly use them. However, there are many > > Android devices using filesystems that do not support checkpoints, so we need > > a generic solution. Here we had two options. One was to use overlayfs to > > manage the changes, then on merge have a script that copies the files to the > > underlying fs. This was rejected on the grounds of compatibility concerns and > > managing the merge through reboots, though it is definitely a plausible > > strategy. The second was to work at the block layer. > > > > At the block layer, dm-snap would have given us a ready-made solution, except > > that there is no sufficiently large spare partition on Android devices. But in > > general there is free space on userdata, just scattered over the device, and > > of course likely to get modified as soon as userdata is written to. We also > > decided that the merge phase was a high risk component of any design. Since > > the normal path is that the update succeeds, we anticipate merges happening > > 99% of the time, and we want to guarantee their success even in the event of > > unexpected failure during the merge. Thus we decided we preferred a strategy > > where the device is in the committed state at all times, and rollback requires > > work, to one where the device remains in the original state but the merge is > > complex. > > What about allocating a big file, using the FIEMAP ioctl to find the > physical locations of the file, creating a dm device with many linear > targets to map the big file and using it as a snapshot store? I think it > would be way easier than re-implementing the snapshot functionality in a > new target. libdevmapper already has code to handle enumerating physical file extents via the dm-stats file mapping support. It should be fairly easy to adapt this to create dm tables rather than dm-stats regions. See dm_stats_create_regions_from_fd() and _stats_map_file_regions(). Bryn.
