← Back to context

Comment by gumby

2 years ago

The whole point of explicit flush is to tell the drive that you want the write at the expense of performance. Either the drive should not accept the flush command or it should fulfill it, not lie.

(BTW this points out the crappy use of the word “performance” in computing to mean nothing but “speed”. The machine should “perform” what the user requests — if you hired someone to do a task and they didn’t do it, we’d say they failed to perform. That’s what’s going on here.)

12 comments

gumby

Reply
kmxdm  2 years ago

The more dire problem is the case where the drive runs out of physical capacity before logical capacity. If the host flushes data that is smaller than the physical write unit of the SSD, capacity is lost to padding (if the SSD honors every Flush). A "reasonable" amount of Flush would not make too much of a difference, but a pathological case like flush-after-every-4k would cause the SSD to run out of space prematurely. There should be a better interface to handle all this, but the IO stack would need to be modified to solve what amounts to a cost issue at the SSD level. It's a race to the bottom selling 1TB consumer SSDs for less than $100.

  • nolist_policy  2 years ago

    I still don't think this is the problem, the drive can just slow down accepting writes until it has reclaimed enough space.

    The bigger problem is manufacturers chasing the performance. Generally you get the feeling they just hit their firmware with a hammer so it barely doesn't break NTFS.

    See also the drama around btrfs' "unreliability", which is all traced back to drives with broken firmware. I fully expect bcachefs will get exactly the same problems.

    • kmxdm  2 years ago

      Yeah, but then you have a write amplification problem. Padding is write amplification from the start, and then GC is invoked many more times than it otherwise would be. There is a fundamental problem with (truly) flushing an IO that is smaller than the media write unit. It will cause problems if "abused." The SSD either needs to take on the cost of mitigation (e.g. caps) or it needs some way to provide hints to the host that don't exist today.

      7 replies →