This is the current queue of things that I've been working on. The main thing these patches are doing is separating out the delayed refs reservations from the global reserve into their own block rsv. We have been consistently hitting issues in production where we abort a transaction because we run out of the global reserve either while running delayed refs or while updating dirty block groups. This is because the math around global reserves is made up bullshit magic that has been tweaked more and more throughout the years. The result is something that is inconsistent across the board and sometimes wrong. So instead we need a way to know exactly how much space we need to keep around in order to satisfy our outstanding delayed refs and our dirty block groups.
Since we don't know how many delayed refs we need at the start of any modification we simply use the nr_items passed into btrfs_start_transaction() as a guess for what we may need. This has the side effect of putting more pressure on the ENOSPC system, but it's pressure we can deal with more intelligently because we always know how much space we have outstanding, instead of guessing with weird global reserve math. This works similar to every other reservation we have, we reserve the worst case up front, and then at transaction end time we free up any space we didn't actually use for delayed refs. My performance tests show that we are bit faster now since we can do more intelligent flushing and don't have to fall back on simply committing the transaction in hopes that we have enough space for everything we need to do. That leads me to the 2nd part of this pull, there's a bunch of fixes around ENOSPC. Because we are a bit faster now there were a bunch of things uncovered in testing, but they seem to be all resolved now. The final chunk of fixes are around transaction aborts. There were a lot of accounting bugs I was running into while running generic/435, so I fixed a bunch of those up so now it runs cleanly. I have been running these patches through xfstests on multiple machines for a while, they are pretty solid and ready for wider testing and review. Thanks, Josef
