On Fri, Aug 19, 2016 at 12:52:21PM +0100, Wols Lists wrote: > On 18/08/16 05:01, Chris Dunlop wrote: >> I'm interested to see if there's a way of essentially doing the above on a >> live system, assuming there's appropriate care taken to not trash any >> existing data (including superblocks). >> >> I.e. is it *theoretically* possible to write the same data back to the whole >> disk safely. E.g. using 'dd' from/to the same disk is almost there, but, as >> described, there's a window of opportunity where you could get stale data on >> the disk and a raid repair could then copy that stale data to the good disk. > > There is something called "scrub". My superficial knowledge of raid > doesn't let me know what it is, but as far as I can make out it forces a > whole-disk-write or somesuch. Explicitly to flush out such problems. If > someone else can tell you how to scrub your disks, I'd try that. A scrub will read the RAID members to check that both sides match (raid 1, 10), or that the checksum is correct (raid 4,5,6). To initiate a scrub of md0: echo repair > /sys/block/md0/md/sync_action You can watch it using /proc/mdstat, e.g.: watch cat /proc/mdstat It won't write anything if it doesn't detect any errors. In my case, I want it to write everything. If I do my 'dd' to write everything as previously described, with the window of opportunity for stale data to end up on the written disk, one option would to run a scrub / repair to check the data is the same - but if I'm unlucky with my dd and the data isn't the same for some sector[s], I want to ensure the correct data is copied over the stale data and not the other way around, e.g. to specify "in the event of a mismatch, use the data from sda and overwrite the data on sdb". Unfortunately I don't know how that can be done. Does anyone know? Cheers, Chris -- 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
