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Comment by Tagbert | Hacker News Reader

In the last couple of version, updates are no longer differentials but full OS updates. They are shipped as a single signed package that is loaded into one of the protected volumes of the disk. This is supposed to be a security measure to prevent compromises of components of the OS. The downside is big slow updates. Hopefully they work on that and do more like the differential updates that they use for iOS.

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They could still apply a tiny patch and then checksum the whole thing, surely?

Just divide the whole thing into read-only units and reuse the unchanged ones, a la nixos.

And if the checksum fails, what then?

Then fall back to downloading the full OS image. Then the worst case would be about the same as today, while the common (99.9%+) case would be fast and bandwidth-cheap.

(Optimization: hash tree, figure out which specific files are broken and download only those.)

Then abort the update? I'm not objecting to A/B partitions or filesystem snapshots or boot environments or however you want to implement it, I'm just saying that you can reconstruct the install image on the machine rather than downloading the whole thing over the wire.

Use a Merkle tree.

Maybe checksum before updating? Or revert the patch if it fails?

To be fair, iOS updates also feel ancient compared to the Android live-update method which just requires a slightly longer reboot. On Android, I sometimes ended up on major releases and didn’t even notice because it just showed up upon my next reboot. On iOS I’m stuck without a phone for 10-15 minutes, which isn’t usually a problem when planned but feels rather archaic.

To emphasize, this isn’t really a serious issue for most users, I’d imagine. I just wouldn’t want to see this as the model for improved Mac updates.

iOS updates are not exactly fast either. I've never seen latest iPad or iPhone updating in less than ~10 minutes, even for minor changes with the ~200-300 Mb update size.

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