I have done experiments on what disks do on power failure, but barriers or turning off disk write cacheing should help against these reorderings. I have also seen disks that destroy old data and the low-level formatting on power failure. And there are other modes in which you can lose data, so having a good backup is a good idea in any case.
Sure, if we are ready to restore our data from backup every time there is a power failure or OS crash, we can use file systems like tmpfs and ext4 for these data. But many of us want to avoid that hassle in the common case when the disk behaves properly, and we need a file system for that case that behaves properly, too. And just like IBM (now Hitachi) and Maxtor (now Seagate) drives are on my don't-buy list after the problems mentioned above, ext4 is on my don't-use list.
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