On Mon, 14 Dec 2015 03:59:18 PM Christoph Anton Mitterer wrote: > I've had some discussions on the list these days about not having > checksumming with nodatacow (mostly with Hugo and Duncan). > > They both basically told me it wouldn't be straight possible with CoW, > and Duncan thinks it may not be so much necessary, but none of them > could give me really hard arguments, why it cannot work (or perhaps I > was just too stupid to understand them ^^)... while at the same time I > think that it would be generally utmost important to have checksumming > (real world examples below).
My understanding of BTRFS is that the metadata referencing data blocks has the checksums for those blocks, then the blocks which link to that metadata (EG directory entries referencing file metadata) has checksums of those. For each metadata block there is a new version that is eventually linked from a new version of the tree root. This means that the regular checksum mechanisms can't work with nocow data. A filesystem can have checksums just pointing to data blocks but you need to cater for the case where a corrupt metadata block points to an old version of a data block and matching checksum. The way that BTRFS works with an entire checksumed tree means that there's no possibility of pointing to an old version of a data block. The NetApp published research into hard drive errors indicates that they are usually in small numbers and located in small areas of the disk. So if BTRFS had a nocow file with any storage method other than dup you would have metadata and file data far enough apart that they are not likely to be hit by the same corruption (and the same thing would apply with most Ext4 Inode tables and data blocks). I think that a file mode where there were checksums on data blocks with no checksums on the metadata tree would be useful. But it would require a moderate amount of coding and there's lots of other things that the developers are working on. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/ -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
