Comment by monocasa

4 years ago

Relevant recent discussion about Apple's NVMe being very slow on FLUSH.

Apple's custom NVMes are amazingly fast – if you don't care about data integrity

https://news.ycombinator.com/item?id=30370551

60 comments

monocasa

Reply
hughrr  4 years ago

Not really a problem when your computer has a large UPS built into it. Desktop macs, not so good.

But really isn’t the point of a journaling file system to make sure it is consistent at one guaranteed point in time, not necessarily without incidental data loss.

  • monocasa  4 years ago

    Journaling file systems depend on commands like FLUSH to create the appropriate barriers to build their journal's semantics.

    • jnwatson  4 years ago

      From the filesystem's perspective looking at storage, the flush does indeed preserve the semantics. It is just that you can't rely on the contents of anything if the power goes out.

    • rowanG077  4 years ago

      I don't have a clue how a journaling FS works. But any ordering should not be observable unless you have a power outage. Can you give an example how a journaling FS could observe something that should be observable?

      15 replies →

  • dathinab  4 years ago

    > Not really a problem when your computer has a large UPS built into it.

    Actually it is (through a small one) to name some examples where it can still lose without full sync:

    - OS crashes

    - random hard reset, e.g. due to bit flips due to e.g. cosmic radiation (happens). Or someone putting their magnetic earphone cases or similar on your laptop or similar.

    Also any application which care about data integrity will do full syncs and in turn will get hit by a huge perf. penalty.

    I have no idea why people are so adamant to defend Apple in this case, it's pretty clear that they messed up as performance with full flush is just WAY to low and this affects anything which uses full flushes, which any application should at least do on (auto-)safe.

    The point of a journalism file system is about making it less likely the file system _itself_ isn't corrupted. Not that the files are not corrupted if they don't use full sync!

    • marginalia_nu  4 years ago

      I had an NVMe controller randomly reset itself a few days ago. I think it was a heat issue. Not really sure though, may be that the motherboard is dodgy.

      This shit does happen.

  • colanderman  4 years ago

    Hard drive write caches are supposed to be battery-backed (i.e., internal to the drive) for exactly this reason. (Apparently the drives tested are not.) Data integrity should not be dependent on power supply (UPS or not) in any way; it's unnecessary coupling of failure domains (two different domains nonetheless -- availability vs. integrity).

    • marcan_42  4 years ago

      The entire point of the FLUSH command is to flush caches that aren't battery backed.

      Battery-backed drives are free to ignore such commands. Those that aren't need to honor them. That's the point.

      Battery- or capacitor-backed enterprise drives are intended to give you more performance by allowing the drive and indeed the OS to elide flushes. They aren't supposed to give you more reliability if the drive and software are working properly. You can achieve identical reliability with software that properly issues flush requests, assuming your drive is honoring them as required by the NVMe spec.

      2 replies →

    • oceanplexian  4 years ago

      As a systems engineer, I think we should be careful throwing words around like “should”. Maybe the data integrity isn’t something that’s guaranteed by a single piece of hardware but instead a cluster or a larger eventually consistent system?

      There will always be trade-offs to any implementation. If you’re just using your M2 SSD to store games downloaded off Steam I doubt it really matters how well they flush data. However if your financial startup is using then without an understanding of the risks and how to mitigate them, then you may have a bad time.

      1 reply →

    • nomel  4 years ago

      > it's unnecessary coupling

      I think much improved write performance is a good example of how it can be beneficial, with minimal risk.

      Everything can be nice ideals of abstraction, until you want to push the envelope.

      1 reply →

  • joenathanone  4 years ago

    UPS won't help if kernel panics.

    • colanderman  4 years ago

      It doesn't need to, kernel panic alone does not cause acknowledged data not to be written to the drive.

      UPS is not perfect though, it's better if your data integrity guarantees are valid independent of power supply. All that requires is that the drive doesn't lie.

  • withinboredom  4 years ago

    > Not really a problem when your computer has a large UPS built into it.

    Except that _one time_ you need to work until the battery fails to power the device, at 8%, because the battery's capacity is only 80%. Granted, this is only after a few years of regular use...

    • monocasa  4 years ago

      In Apple's defense, they probably have enough power even in the worst case to limp along enough to flush in laptop form factor, even if the power management components refuse to power the main CCXs. Speccing out enough caps in the desktop case would be very Apple as well.

      14 replies →

marcan_42  4 years ago

And the two vendors I tested as faster than Apple are precisely the two vendors OP found to be reliable, so my findings still stand.