store: enable deltas for core devices too

[mvo5]

This enables delta refreshes everywhere.

[zyga]

Are we shipping the required binary in core yet? I don't see it in my system (I'm tracking edge).

[ogra1]

yes we do ...

ogra@stream:~$ sudo find / -name '*xdelta*'|grep ^/snap/core|grep bin
/snap/core/3965/usr/bin/xdelta3
/snap/core/3750/usr/bin/xdelta3
/snap/core/3884/usr/bin/xdelta3
ogra@stream:~$ uname -m
armv7l

[zyga]

Ah. I searched using find /snap/core/current -name xdelta3 and that finds nothing (I need an extra trailing slash).

All good!

[zyga]

@mvo5 unit tests are failing:

FAIL: store_test.go:3921: remoteRepoTestSuite.TestUbuntuStoreRepositoryDefaultsDeltasOnClassicOnly

store_test.go:3934:
    c.Check(r.Header.Get("X-Ubuntu-Delta-Formats"), Equals, t.deltaFormatStr)
... obtained string = "xdelta3"
... expected string = ""

[mvo5]

@zyga yeah, sorry for this, just noticed and forced push the fix. If I had seen your comment early enough I would have pushed it as a separate commit.

[mvo5]

I set this to blocked because of: https://forum.snapcraft.io/t/refresh-causes-download-of-full-snap-file-instead-of-delta/3470/15 - I'm not sure what the best way forward is here, maybe we can contact them via our CE team?

[ogra1]

we should really ask them to add the variable to /etc/environment for a test (as i mentioned in the thread i'll do this over the weekend on one customer device here (200MHz single core cpu, 256M RAM) to see the impact).

IIRC when i last measured it for @chipaca on a beaglebone the difference was like:

x86 multi core CPU -> below 1sec to apply a core delta
armhf single core (beagle) CPU -> something like 8-10 sec or so for the same task

and IIRC the worry was that this extrapolates as your snaps get bigger ... though i guess it is not very likely that you run 1GB sized snaps on such HW after all ...

also, as long as we make sure we do not leak this out of edge we should be fine ...

[codecov-io]

Codecov Report

Merging #4639 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4639      +/-   ##
==========================================
- Coverage   78.82%   78.81%   -0.01%     
==========================================
  Files         470      470              
  Lines       34072    34071       -1     
==========================================
- Hits        26857    26853       -4     
- Misses       5048     5050       +2     
- Partials     2167     2168       +1

Impacted Files	Coverage Δ
store/store.go	80.71% <100%> (-0.02%)	⬇️
cmd/snap-seccomp/main.go	52.6% <0%> (-0.53%)	⬇️
overlord/snapstate/snapstate.go	81.12% <0%> (-0.21%)	⬇️

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[mvo5]

Alex was kind enough to do some tests: https://forum.snapcraft.io/t/refresh-causes-download-of-full-snap-file-instead-of-delta/3470/19 and it looks like even on low end arm hardware the time to apply the delta is very reasonable. I will remove the blocked label and I think we should do this for 2.33 (which will give us plenty of time to test this in edge)