reiser4 is fully atomic. A transactions happens completely or it doesn't. Unlike ext4 or btrfs or xfs.
reiser4 also uses barriers (the others use them too, but), when barriers are not available for some reason or another, it complains in dmesg and goes into sync mode. This combined makes it pretty robust against power failures. You won't get the good old xfs/ext4/btrfs problem that a rename can end with two useless empty files.
