Re: dm-snapshot for system updates in Android

[Mikulas Patocka <mpatocka@redhat.com>]

Hi

On Fri, 25 Oct 2019, Alessio Balsini wrote:

> Hello everyone!
> 
> I hope you will appreciate knowing that we are currently evaluating the use of
> dm-snapshot to implement a mechanism to obtain revertible, space-efficient
> system upgrades in Android.  More specifically, we are using
> dm-snapshot-persistent to test the updated device after reboot, then issue a
> merge in case of success, otherwise, destroy the snapshot.
> This new update mechanism is still under evaluation, but its development is
> openly done in AOSP.
> 
> At the current stage, we have a prototype we are happy with, both in terms of
> space consumption overhead (for the COW device) and benchmarking results for
> read-write and merge operations.
> 
> I would be glad if you could provide some feedback on a few points that I don't
> have completely clear.
> 
> 
> -- Interface stability
> 
> To obtain an initial, empty COW device as quick as possible, we force to 0 only
> its first 32 bit (magic field). This solution looks clear from the kernel code,
> but can we rely on that for all the kernels with SNAPSHOT_DISK_VERSION == 1?

It will work, but, to be consistent with lvm, I suggest to overwrite the 
first 4k with zeroes.

> Would you appreciate it if a similar statement is added as part of
> /Documentation, making this solution more stable? Or maybe I can think of
> adding an initialization flag to the dm-snapshot table to explicitly request
> the COW initialization within the kernel?
> 
> Another issue we are facing is to be able to know in advance what the minimum
> COW device size would be for a given update to be able to allocate the right

This is hard to say, it depends on what the user is doing with the phone. 
When dm-snapshot runs out of space, it invalidates the whole snapshot. 
You'll have to monitor the snapshot space very carefully and take action 
before it fills up.

I suggest - run main system on the origin target and attach a snapshot 
that will be used for backup of the data overwritten in the origin. If the 
updated system fails, merge the snapshot back into the origin; if the 
update succeeds, drop the snapshot. If the user writes too much data to 
the device, it would invalidate the only the snapshot (so he can't revert 
anymore), but it would not invalidate the origin and the data would not be 
lost.

> size for the COW device in advance.  To do so, we rely on the current COW
> structure that seems to have kept the same stable shape in the last decade, and
> compute the total COW size by knowing the number of modified chunks. The
> formula would be something like that:
> 
>   table_line_bytes      = 64 * 2 / 8;
>   exceptions_per_chunk  = chunk_size_bytes / table_line_bytes;
>   total_cow_size_chunks = 1 + 1 + modified_chunks
>                         + modified_chunks / exceptions_per_chunk;
> 
> This formula seems to be valid for all the recent kernels we checked. Again,
> can we assume it to be valid for all the kernels for which
> SNAPSHOT_DISK_VERSION == 1?

Yes, we don't plan to change it.

> -- Alignment
> 
> Our approach follows the solution proposed by Mikulas [1].
> Being the block alignment of file extents automatically managed by the
> filesystem, using FIEMAP should have no alignment-related performance issue.
> But in our implementation we hit a misalignment [2] branch which leads to
> dmwarning messages [3, 4].
> 
> I have a limited experience with the block layer and dm, so I'm still
> struggling in finding the root cause for this, either in user space or kernel
> space.

I don't know. What is the block size of the filesystem? Are all mappings 
aligned to this block size?

> But our benchmarks seems to be good, so we were thinking as last option to
> rate-limit or directly remove that warning from our kernels as a temporary
> solution, but we prefer to avoid diverging from mainline. Rate-limiting is a
> solution that would make sense also to be proposed in the list, but completely
> removing the warning doesn't seem the right thing to do. Maybe we are
> benchmarking something else? What do you think?
> 
> Many thanks for taking the time to read this, feedbacks would be highly
> appreciated.
> 
> Regards.
> Alessio
> 
> [1] https://www.redhat.com/archives/dm-devel/2018-October/msg00363.html
> [2] https://elixir.bootlin.com/linux/v5.3/source/block/blk-settings.c#L540
> [3] https://elixir.bootlin.com/linux/v5.3/source/drivers/md/dm-table.c#L484
> [4] https://elixir.bootlin.com/linux/v5.3/source/drivers/md/dm-table.c#L1558

Mikulas