Neil Brown wrote: >> Is there any way to tell MD to do verify-on-write and >> read-from-all-disks on a RAID1 array? > > No. > I would have thought that modern disk drives did some sort of > verify-on-write, else how would they detect write errors, and they are > certainly in the best place to do verify-on-write. Really? My guess was that they wouldn't, because it would lead to less performance. And that's why read errors crop up at read time. > Doing it at the md level would be problematic as you would have to > ensure that you really were reading from the media and not from some > cache somewhere in the data path. I doubt it would be a mechanism > that would actually increase confidence in the safety of the data. Hmm. Could hack it by reading / writing blocks larger than the cache. Ugly. > Imagine a filesystem that could access multiple devices, and where it > kept index information it didn't just keep one block address, but > rather kept two block address, each on different devices, and a strong > checksum of the data block. This would allow much the same robustness > as read-from-all-drives and much lower overhead. As in, "if the checksum fails, try loading the data blocks [again] from the other device"? Not sure why a checksum of X data blocks should be cheaper performance-wise than a comparison between X data blocks, but I can see the point in that you only have to load the data once and check the checksum. Not quite the same security, but almost. > In summary: > - you cannot do it now. > - I don't think md is at the right level to solve these sort of problems. > I think a filesystem could do it much better. (I'm working on a > filesystem .... slowly...) > - read-from-all-disks might get implemented one day. verify-on-write > is much less likely. > >> Apologies if the answer is in the docs. > > It isn't. But it is in the list archives now.... Thanks! :-) (Guess I'll drop the idea for the time being...) - 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
