On 11/14/2014 04:51 PM, Hugo Mills wrote:
Chris, Josef, anyone else who's interested,On IRC, I've been seeing reports of two persistent unsolved problems. Neither is showing up very often, but both have turned up often enough to indicate that there's something specific going on worthy of investigation. One of them is definitely a btrfs problem. The other may be btrfs, or something in the block layer, or just broken hardware; it's hard to tell from where I sit. Problem 1: ENOSPC on balance This has been going on since about March this year. I can reasonably certainly recall 8-10 cases, possibly a number more. When running a balance, the operation fails with ENOSPC when there's plenty of space remaining unallocated. This happens on full balance, filtered balance, and device delete. Other than the ENOSPC on balance, the FS seems to work OK. It seems to be more prevalent on filesystems converted from ext*. The first few or more reports of this didn't make it to bugzilla, but a few of them since then have gone in. Problem 2: Unexplained zeroes Failure to mount. Transid failure, "expected xyz, have 0". Chris looked at an early one of these (for Ke, on IRC) back in September (the 27th -- sadly, the public IRC logs aren't there for it, but I can supply a copy of the private log). He rapidly came to the conclusion that it was something bad going on with TRIM, replacing some blocks with zeroes. Since then, I've seen a bunch of these coming past on IRC. It seems to be a 3.17 thing. I can successfully predict the presence of an SSD and -odiscard from the "have 0". I've successfully persuaded several people to put this into bugzilla and capture btrfs-images. btrfs recover doesn't generally seem to be helpful in recovering data. I think Josef had problem 1 in his sights, but I don't know if additional images or reports are helpful at this point. For problem 2, there's obviously something bad going on, but there's not much else to go on -- and the inability to recover data isn't good. For each of these, what more information should I be trying to collect from any future reporters?
So for #2 I've been looking at that the last two weeks. I'm always paranoid we're screwing up one of our data integrity sort of things, either not waiting on IO to complete properly or something like that. I've built a dm target to be as evil as possible and have been running it trying to make bad things happen. I got slightly side tracked since my stress test exposed a bug in the tree log stuff an csums which I just fixed. Now that I've fixed that I'm going back to try and make the "expected blah, have 0" type errors happen.
As for the ENOSPC I keep meaning to look into it and I keep getting distracted with other more horrible things. Ideally I'd like to reproduce it myself, so more info on that front would be good, like do all reports use RAID/compression/some other odd set of features? Thanks for taking care of this stuff Hugo, #2 is the worst one and I'd like to be absolutely sure it's not our bug, once I'm happy we aren't I'll look at the balance thing.
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