So, in my case at least, I'd guess that dropping the kernel from 4.16 to 4.4 combined with a failed disk is what the root cause was. I've done what little recovery I can of the current state of files using btrfs restore. Is there a means of rebuilding the metadata using the existing data on the drives? Can I put that metadata elsewhere in a different location so not to overwrite anything? I'm thinking of moving onto destructive recovery at this point anyway. Cheers, Justin On Wed, May 20, 2020 at 11:20 PM Zygo Blaxell <ce3g8jdj@xxxxxxxxxxxxxxxxxxxxx> wrote: > > On Wed, May 20, 2020 at 10:53:19PM +0200, Johannes Hirte wrote: > > On 2020 Mai 19, Zygo Blaxell wrote: > > > > > > Corollary: Never use space_cache=v1 with raid5 or raid6 data. > > > space_cache=v1 puts some metadata (free space cache) in data block > > > groups, so it violates the "never use raid5 or raid6 for metadata" rule. > > > space_cache=v2 eliminates this problem by storing the free space tree > > > in metadata block groups. > > > > > > > This should not be a real problem, as the space-cache can be discarded > > and rebuild anytime. Or do I miss something? > > Keep in mind that there are multiple reasons to not use space_cache=v1; > space_cache=v1 is quite slow, especially on filesystems big enough that > raid5 is in play, even when it's not recovering from integrity failures. > > The free space cache (v1) is stored in nodatacow inodes, so it has all > the btrfs RAID data integrity problems of nodatasum, plus the parity > corruption and write hole issues of raid5. Free space tree (v2) is > stored in metadata, so it has csums to detect data corruption and transid > checks for dropped writes, and if you are using raid1 metadata you also > avoid the parity corruption bug in btrfs's raid5/6 implementation and > the write hole. v2 is faster too, especially at commit time. > > The probability of undetected space_cache=v1 failure is low, but not zero. > In the event of failure, the filesystem should detect the error when it > tries to create new entries in the extent tree--they'll overlap existing > allocated blocks, and the filesystem will force itself read-only, so > there should be no permanent damage other than killing any application > that was writing to the disk at the time. > > Come to think of it, though, the space_cache=v1 problems are not specific > to raid5. You shouldn't use space_cache=v1 with raid1 or raid10 data > either, for the same reasons. > > In the raid5/6 case it's a bit simpler: kernels that can't do > space_cache=v2 (4.4 and earlier) don't have working raid5 recovery either. > > > -- > > Regards, > > Johannes Hirte > > -- Justin Engwer Mautobu Business Services 250-415-3709
