On Tue, Aug 15, 2017 at 08:44:16PM -0500, Vijay Chidambaram wrote: > Hi Amir, > > I neglected to mention this earlier: CrashMonkey does not require > recompiling the kernel (it is a stand-alone kernel module), and has > been tested with the kernel 4.4. It should work with future kernel > versions as long as there are no changes to the bio structure. > > As it is, I believe CrashMonkey is compatible with the current kernel. > It certainly provides functionality beyond log-writes (the ability to > replay a subset of writes between FLUSH/FUA), and we intend to add > more functionality in the future. > > Right now, CrashMonkey does not do random sampling among possible > crash states -- it will simply test a given number of unique states. > Thus, right now I don't think it is very effective in finding > crash-consistency bugs. But the entire infrastructure to profile a > workload, construct crash states, and test them with fsck is present. > > I'd be grateful if you could try it and give us feedback on what make > testing easier/more useful for you. As I mentioned before, this is a > work-in-progress, so we are happy to incorporate feedback. > Sorry I was travelling yesterday so I couldn't give this my full attention. Everything you guys do is already accomplished with dm-log-writes. If you look at the example scripts I've provided https://github.com/josefbacik/log-writes/blob/master/replay-individual-faster.sh https://github.com/josefbacik/log-writes/blob/master/replay-fsck-wrapper.sh The first initiates the replay, and points at the second script to run after each entry is replayed. The whole point of this stuff was to make it as flexible as possible. The way we use it is to replay, create a snapshot of the replay, mount, unmount, fsck, delete the snapshot and carry on to the next position in the log. There is nothing keeping us from generating random crash points, this has been something on my list of things to do forever. All that would be required would be to hold the entries between flush/fua events in memory, and then replay them in whatever order you deemed fit. That's the only functionality missing from my replay-log stuff that CrashMonkey has. The other part of this is getting user space applications to do more thorough checking of consistency that it expects, which I implemented here https://github.com/josefbacik/fstests/commit/70d41e17164b2afc9a3f2ae532f084bf64cb4a07 fsx will randomly do operations to a file, and every time it fsync()'s it saves it's state and marks the log. Then we can go back and replay the log to the mark and md5sum the file to make sure it matches the saved state. This infrastructure was meant to be as simple as possible so the possiblities for crash consistency testing were endless. One of the next areas we plan to use this in Facebook is just for application consistency, so we can replay the fs and verify the application works in whatever state the fs is at any given point. I looked at your code and you are logging entries at submit time, not completion time. The reason I do those crazy acrobatics is because we have had bugs in previous kernels where we were not waiting for io completion of important metadata before writing out the super block, so logging only at completion allows us to catch that class of problems. The other thing CrashMonkey is missing is DISCARD support. We fuck up discard support constantly, and being able to replay discards to make sure we're not discarding important data is very important. I'm not trying to shit on your project, obviously it's a good idea, that's why I did it years ago ;). The community is going to use what is easiest to use, and modprobe dm-log-writes is a lot easier than compiling and insmod'ing an out of tree driver. Also your driver won't work on upstream kernels because of the way the bio flags were changed recently, which is why we prefer using upstream solutions. If you guys want to get this stuff used then it would be better at this point to build on top of what we already have. Just off the top of my head we need 1) Random replay support for replay-log. This is probably a day or two worth of work for a student. 2) Documentation, because right now I'm the only one who knows how this works. 3) My patches need to actually be pushed into upstream fstests. This would be the largest win because then all the fs developers would be running the tests by default. 4) Multi-device support. One thing that would be good to have and is a dream of mine is to connect multiple devices to one log, so we can do things like make sure mdraid or btrfs's raid consistency. We could do super evil things like only replay one device, or replay alternating writes on each device. This would be a larger project but would be super helpful. Thanks, Josef
