From: "Bryn M. Reeves" <email@example.com> To: Mikulas Patocka <firstname.lastname@example.org> Cc: Paul Lawrence <email@example.com>, Mike Snitzer <firstname.lastname@example.org>, Jonathan Corbet <email@example.com>, Shaohua Li <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, Alasdair Kergon <firstname.lastname@example.org> Subject: Re: [dm-devel] [RFC] dm-bow working prototype Date: Thu, 25 Oct 2018 11:20:40 +0100 [thread overview] Message-ID: <20181025102039.GA9378@localhost.localdomain> (raw) In-Reply-To: <alpine.LRH.email@example.com> 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.
next prev parent reply other threads:[~2018-10-25 10:21 UTC|newest] Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-10-23 21:23 Paul Lawrence 2018-10-23 22:18 ` Alasdair G Kergon 2018-10-24 18:42 ` Paul Lawrence 2018-10-24 19:24 ` [dm-devel] " Mikulas Patocka 2018-10-25 0:01 ` Alasdair G Kergon 2018-10-25 10:20 ` Bryn M. Reeves [this message] 2018-10-25 17:23 ` Paul Lawrence 2018-10-26 20:03 ` Mikulas Patocka 2018-10-29 16:51 ` Paul Lawrence 2018-11-15 23:15 ` Mikulas Patocka 2018-12-02 10:07 ` Sandeep Patil 2018-10-25 16:30 ` MegaBrutal 2018-10-25 18:13 ` Paul Lawrence 2018-10-25 21:15 ` Wols Lists 2018-10-25 21:43 ` [dm-devel] " Darrick J. Wong
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