Currently we use writeback caching for disks
on our test vms, however that honors syncs and
IO could become blocked/locked if the sync has
not completed with the double virtualization that
occurs in CI.
In theory, we don't need to worry about the
buffer cache behavior for our test VMs.
Overall, it appears that there is a performance
gain of about 10-15% with making this change because
we're not honoring syncs to not lock-up IO.
Change-Id: Ib09dd7fd41c6b1cbc0e2b707bfc9e4ff8dfa6fad
afa611c4e5