I am jumping into this thread late, and maybe this has already been
stated clearly, but from my experience benchmarking, LVM does *not*
lie about fsync() on the servers I've configured. An fsync() goes to
the physical device. You can see it clearly by setting the write
cache on the RAID controller to write-through policy. Performance
decreases to what the disks can do.
And my colleagues and clients have tested yanking the power plug and
checking that the data got to the RAID controller's battery-backed
cache, many many times. In other words, the data is safe and durable,
even on LVM.
However, I have never tried to do this on volumes that span multiple
physical devices, because LVM can't take an atomic snapshot across
them, which completely negates the benefit of LVM for my purposes. So
I always create one logical disk in the RAID controller, and then
carve that up with LVM, partitions, etc however I please.
I almost surely know less about this topic than anyone on this thread.
Baron