On Mon, Aug 13 2018, David C. Rankin wrote: > 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. NeilBrown
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