[CentOS] extremely slow NFS performance under 6.x [SOLVED]

I've posted here about this a number of times. The other admin I work with
had been playing with it recently, with some real problems we'd been
having, and this time, with a year or so's more stuff to google, and newer
documentation, found the problem.

What we'd been seeing: cd to an NFS-mounted directory, and from an
NFS-mounted directory, tar -xzvf a 25M or so tar.gz, which unpacks to
about 105M. Under CentOS 5, on a local drive, seconds; doing the above,
about 35 seconds. Mount options included sync. Under 6.x, from the
beginning, it was 6.5 to 7 *minutes*.

The result was that we'd been keeping our home directory servers on 5.

What he found was the mount option barrier. According to one or two hits I
found, googling, it's not clear that 5.x even recognizes this option. From
upstream docs, 
<https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/writebarrieronoff.html>,
it's enabled by default, and affects *all* journalled filesystems.

Remounting the drive with -o nobarrier, I just NFS mounted an exported
directory... and it took 20 seconds.

Since most of our systems are all on UPSes, we're not worried about sudden
power loss... and my manager did a jig, and we're starting to talk about
migrating the rest of our home directory servers....

        mark