I have two 3TB disks in a software RAID 1 setup and the host OS is 64bit Debian wheezy.
dd if=/dev/zero of=test bs=64k count=3k oflag=direct && rm test
201326592 bytes (201 MB) copied, 1.423 s, 141 MB/s
If I alter the dd command to use synchronized IO calls (by changing the oflag switch from "direct" to "sync"), write peformance drops through the floor:
201326592 bytes (201 MB) copied, 76.0286 s, 2.6 MB/s
Obviously, synced IO results in a performance hit but I was expecting write throughput to drop to maybe half or a third (worst case) of the direct equivalent. 2.6MB/s seems extreme and makes me think there's a problem somewhere.