On Sat, Aug 20 2016, Chris Dunlop wrote: > Hi Neil, > > Nice work on the Bus1 article! Thanks :-) > > On Sat, Aug 20, 2016 at 07:26:27AM +1000, NeilBrown wrote: >> On Fri, Aug 19 2016, Chris Dunlop wrote: >>> 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? >> >> If it is the second device in the array (as listed by mdadm --detail) >> then you can stop the array and re-assemble with --update=resync. > > That's nearly there - except in this specific case it's my root filesystem > so I can't stop the array without booting into a recovery disk etc. Of > course I could do that, but the point of the exercise is to see if it can > be done live, safely. Well... you could cd /sys/block/mdXX/md echo frozen > sync_action echo 0 > resync_start echo idle > sync_action that should start a resync on a live array. Still, only works for non-first device in RAID1 > >> If it is the first device I can only suggest that you >> fail the device and add it again: >> >> mdadm /dev/mdXX --fail /dev/sdYY >> mdadm /dev/mdXX --remove /dev/sdYY >> mdadm /dev/mdYY --add /dev/sdYY >> >> If the "good" drive fails during the rewrite it might be a little bit >> fiddley getting the array working again, but all the data will certainly >> be there on the device you are re-writing, so you won't lose anything. > > OK, that sounds good. What would the process be if the good drive fails, > either completely, or a few specific sectors? If you think there is a serious risk of that happening, then it's best to skip this option. You would need to boot from a rescue disk and re-create the array using just the working device - and make sure the same data-offset and size are used. Certainly possible, but not at all straightforward. Another thing you could do, particularly if you know what region of the device needs to be over-written, is to write sector numbers to suspend_lo and suspend_hi. This will suspend all IO through the /dev/mdXX device to that range of array sectors. Then you could read from/write to the raw device with dd or whatever. raid6check.c does this on a raid6 to correct errors that can be detected with the raid6 syndrome, even while the array is online. A similar thing could be done to allow individual blocks to be rewritten. Care is needed to map between array addresses and device addresses. NeilBrown
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