On Mon, Apr 27, 2015 at 06:18:57PM +1000, Adam Goryachev wrote: > On 27/04/15 17:35, Mikael Abrahamsson wrote: > >On Mon, 27 Apr 2015, David Brown wrote: > > > >>btrfs has data checksums like that. Like Neil, I question the > >>necessity for harddisks, but such checksums are lower cost than > >>reading the data twice from two disks (as they are stored as > >>part of the metadata that you already read), and can offer some > >>protection against serious hardware problems. (Checksums like > >>this cannot easily be implemented in a transparent block device > >>such as md raid - it is more practical to have them as part of > >>the filesystem, as done with btrfs.) > > > >Only way I can imagine this being done would be for instance to > >add a 4KiB block for every 128KiB chunk or something like that, > >and perhaps have a smaller checksum for each 4KiB block within > >that 128KiB chunk. > > > >I doubt anyone would be interested in putting efforts into > >creating this though as it would have "interesting" performance > >drawbacks, and that work is probably better spent by making sure > >that btrfs and/or zfs gets more development/testing than it is to > >put that effort into md. I personally prefer md to be fairly > >"simple" so we have as few bugs as possible in it, I'd say that md > >generally works and the number of developers working heroically on > >its current incarnation is barely enough to make sure that the > >codebase works as well as it must considering the critical > >function it serves for a lot of us. > > > >This has been discussed before and nobody has shown interest in > >actually developing code for it, so we're still at the feature > >request and "brainstorming about design" state, and without actual > >coder(s) willing to actually implement, it's not going to get > >further than this stage. > > > Speaking of which, I'm not convinced that we should spend that > developer time on each and every FS (eg, duplicated effort for > btrfs, zfs, and any others that do the same). It also means you must > remove MD Raid, to allow the FS to directly access each of the > underlying devices. Obviously, there are advantages in both methods. > Yeah, having checksums support in MD-RAID would be very welcome! > As you and others said, without someone willing to implement/write > this feature, then it isn't going to happen. > Yeah this is the problem, someone actually needs to do it :) There actually IS a proof-of-concept checksums support for MD-RAID, but it was never upstreamed, and it was a quick and 'naive' implementation. http://pages.cs.wisc.edu/~bpkroth/cs736/md-checksums/md-checksums-paper.pdf http://pages.cs.wisc.edu/~bpkroth/cs736/md-checksums/ > Regards, > Adam > -- Pasi -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
