NeilBrown <neilb@xxxxxxxx> 于2018年8月14日周二 上午10:53写道： > > On Tue, Aug 14 2018, Jinpu Wang wrote: > > > NeilBrown <neilb@xxxxxxxx> 于2018年8月14日周二 上午1:31写道： > >> > >> 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 > > Thanks for your comments, Neil. > > Convert to raid5 with 2 drives will not only cause perrormance drop, > > will also disable the redundancy. > > It's clearly a no go. > > I don't understand why you think it would disable the redundancy, there > are still two copies of every block. Both RAID1 and RAID5 can survive a > single device failure. I thought RAID5 requirs at least 3 drive with parity, clearly, I was wrong. Sorry. I'm testing the script with raid5, if works as expected. will do some performance test to see the cost raid1 vs raid5. > > I agree about performance and don't expect this would be a useful thing > to do; it just seemed the simplest way to explain the cost that would be > involved in resisting this attack. > > NeilBrown Thanks, Jack
