On Wed, Dec 9, 2009 at 2:53 AM, Mikael Abrahamsson <swmike@xxxxxxxxx> wrote: > Generally, my experience has been that total disk failures are fairly rare, > instead with the much larger disks today, I get single block/sector > failures, meaning 512 bytes (or 4 k, I don't remember) can't be read. Is > there any data to support this? > I agree with this failure mode. I've seen occasional disk failures; usually on single drive systems, far more often are either single failures; or in the case of laptops (and possibly also hard drives running in more seismically active areas) occasional runs of head-crashes. In the case of a head-crash it would be a _VERY_ good idea to copy the data off first then recover it, but one would expect only a moderate volume of poison data. Having a layer to identify which data is suspect, and potentially provide recovery information would be a great idea. In my use cases I'd probably dedicate an entire stripe for every 64 that are backed by it. Changing any information within that 64 stripe section would result in a change to the parity data, but that layer needn't be updated constantly, during idle periods would be a sufficient safety net, so long as it was automated; the list of checksums would of course be updated with the same operation. So there would be three basic functions: 1) Determine if chunks are the expected checksum. 2) Determine if chunks are the correct version (to provide upper layers with atomic storage). 3) Provide low-density recovery data. Not enough to protect against a disk loss, but whatever scale of safety net between 0 and 100% of a drive is desired. -- 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
