>> On 08/11/2018 02:06 AM, NeilBrown wrote: >>> It might be expected behaviour with async direct IO. >>> Two threads writing with O_DIRECT io to the same address could result in >>> different data on the two devices. This doesn't seem to me to be a >>> credible use-case though. Why would you ever want to do that in >>> practice? >>> >>> NeilBrown >> >> My only thought is while the credible case may be weak, if it is something >> that can be protected against with a few conditionals to prevent the different >> data on the slaves diverging -- then it's worth a couple of conditions to >> prevent the nut that know just enough about dd from confusing things.... > >Yes, it can be protected against - the code is already written. >If you have a 2-drive raid1 and want it to be safe against this attack, >simply: > > mdadm /dev/md127 --grow --level=raid5 > >This will add the required synchronization between writes so that >multiple writes to the one block are linearized. There will be a >performance impact. Hi Neil, if I would store all the inflight writes in say an rb-tree by their offsets, look for the offset of each incoming write in the tree and, if it can be found, postpone the write until the one to the same offset returns: would that solve the problem? I mean apart from the performance penalty due to the search, do you think it would cover for the reorder of the writes going to the same sector in theory? Thank you, Danil. P.S. When I try to do mdadm /dev/md127 --grow --level=raid5 on my raid1, I get this: mdadm: Sorry, no reshape for RAID-1! unfreeze Do I need some specific version? What would a raid5 on top of only two drives actually do?
