On Sun, Aug 13, 2017 at 8:16 AM, Goffredo Baroncelli <kreijack@xxxxxxxxx> wrote: > Hi all, > > in the BTRFS wiki, in the status page, in the "line" RAID5/6 it is reported that the parity is not checksummed. This was reported several time in the ML and also on other site (e.g. phoronix) as a BTRFS defect. > > However I was unable to understand it, and I am supposing that this is a false mith. > > So my question is: the fact that in the BTRFS5/6 the parity is not checksummed could be considered a defect ? > > My goal is to verify if there is a rationale to require the parity checksummed, and if no I would like to remove this from the wiki. It is not a per se defect. If parity is corrupt, and parity is needed for reconstruction, reconstruction will be corrupt, but is then detected and we get EIO [1] Further, the error detection of corrupt reconstruction is why I say Btrfs is not subject *in practice* to the write hole problem. [2] [1] I haven't tested the raid6 normal read case where a stripe contains corrupt data strip and corrupt P strip, and Q strip is good. I expect instead of EIO, we get a reconstruction from Q, and then both data and P get fixed up, but I can't find it in comments or code. https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/fs/btrfs/raid56.c?h=v4.12.7 line 1851, I'm not sure where we're at exactly at this line; seems like it must be a scrub because P & Q are not relevant if data is good. [2] Is Btrfs subject to the write hole problem manifesting on disk? I'm not sure, sadly I don't read the code well enough. But if all Btrfs raid56 writes are full stripe CoW writes, and if the prescribed order guarantees still happen: data CoW to disk > metadata CoW to disk > superblock update, then I don't see how the write hole happens. Write hole requires: RMW of a stripe, which is a partial stripe overwrite, and a crash during the modification of the stripe making that stripe inconsistent as well as still pointed to by metadata. -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
