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Comment by marcan_42

4 years ago

You presumably don't reboot your laptop by connecting a USB-PD gadget that issues a hard reset. A normal OS reboot is fine, that will flush the cache.

The most common situation where this would affect laptops, in my experience so far, would be a broken driver causing a kernel lockup (not a panic) which triggers a watchdog reboot. That situation wouldn't allow for an NVMe flush.

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marcan_42

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signal11  4 years ago

For products like the Mac Mini, which don’t have a battery, does this mean that a loss of mains power will cause data loss? Because brownouts do happen occasionally…

  • marcan_42  4 years ago

    Yes. I've tested yanking the power and can easily see 5 seconds of data loss for data that was fsync()ed (but not full synced). I'm not sure yet if corruption due to reordering is also possible, but it seems likely.

rowanG077  4 years ago

Depends what exactly is a hard reboot. I don't reboot my laptop by issueing USB-PD command. But I do by holding the power button.

  • marcan_42  4 years ago

    I just tested that. Holding down the power button invokes a (somewhat special) btn_rst kernel panic before it has a chance to invoke a true hardware reset, and kernel panics involve an NVMe driver hook which I'm pretty sure issues a flush. Should be safe.

    At least re: this issue; it's still a bad idea because it's only safe if all software is written following data integrity and flush rules to the letter, and most software isn't. You're eventually going to run into issues on any OS by doing that, because most software doesn't get this right unless it's a database. And you're still going to lose data that's in buffer cache, I'm pretty sure that won't get flushed.

  • Cthulhu_  4 years ago

    See, that's a forced shutdown, a last resort measure; it's using a sledgehammer to tap in a nail. You shouldn't do that as a habit, even if this particular optimization issue wasn't a thing.

    I mean I grew up diligently turning off my PC by parking the disk and using the various operating system level shutdown procedures. Nowadays I smack the off button, but that still just triggers the OS shutdown procedure. I don't turn my Mac off as a rule, its sleep mode actually works. ish.

  • Thomaschaaf  4 years ago

    Care to explain why?

    • olliej  4 years ago

      If they're like me: outside of a software update I only reboot when the machine is not responding, at which point hard reboot is faster and more robust. I recognize it's not ideal, but I also don't think it's reasonable for the system to ever get to a point where I should be wanting to restart to "fix" it - and I would think it is a serious bug if doing so ever corrupted the system or lost any "saved" data.