Comment by olliej
4 years ago
The whole point of a transactional database is that even in the case of a power loss you do not lose data. If you UPS blows up, and so you lose power, you should not lose data.
The point here is that on the apple systems if you do the correct thing your performance drops to that of spinning disks.
Add a secondary UPS. What will be the next excuse?
It's ridiculous to think that in case of power loss you expect 100% data integrity - it might happen in the middle of the command execution. If the system should be unkillable, it should have an unkillable power source in the first place.
The entire point of modern journaling filesystems and properly designed transactional databases is to ensure 100% data integrity in case of power loss, every time, no matter what. The thinking you have is from the 1990s. We can (and do) do better today.
A properly designed transactional database will only ever "fail ahead". If power fails a transaction that was in the process of committing might commit without an ack, but will never return an ack and then be lost on the next startup. The ack means the data is safe, regardless of what happened afterwards.
After reading this comment I’m not so sure about that: https://news.ycombinator.com/item?id=30371181
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>Add a secondary UPS. What will be the next excuse?
You shouldn't have to add a secondary UPS at all, period, and still get that.
Databases are designed that way (for integrity under sudden power loss) - the OS just needs to provide a standard call for the sync that they can use.
Now, fsync not guaranteeing a write is one thing -- and it's common in other OSes, even Linux behaved like that.
The non-commital fullsync on the other hand (and the slow speed) are problematic, and that's not an excuse for the user having such a bizarro case as wanting to run a DB on their Mac Mini without 2 UPS, that's you excusing Apple.
Not to mention that 2 UPS wont solve the problem if you're not there to shut down the computer gracefully as they, themselves, are depleted (e.g. at night) when there's a powerloss.
Since no Mac device has two power support adding a second UPS means chaining which will only increase the risk of something going wrong in the chain.
Nobody expects 100% data integrity on power of. What is expected is that data that what was fsynced has 100% data integrity once that system call returns. This information is also used when moving files across the network, the file gets deleted on the sender when the receiver said fsync is completed. This means you could loose entire files of data when moving things over the network onto a Mac
For what it's worth, adding an external UPS to a Mac laptop counts as two, and in fact you can add one per Type C port + MagSafe, so you can have up to 5 battery backups for the 2021 Macbook Pro line (internal + MagSafe + 3 x Type C).
It's silly, but you can :-)
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This means the world would be full of complaints from macOS users, but for some reason, we only know about this detail because of that “shocking” Twitter thread.
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