On Wed, Jun 29, 2011 at 07:08:18AM -0500, Anthony Liguori wrote: > As long as we advertise wce and wce can be toggled from the guest, I don't > think the default is all that important. I think cache=on is the right > default for most common use cases.
What do you mean with cache=on? We have cache=[none,writeback,writethrough,unsafe]. As discussed about a dozen times before these are an incomplete choice of the [odirect=on,off; osync=on,off, ignore cache flushes=on,off] matrix. Not using O_DIRECT when it is available is almost always the wrong choice. It means an additional data copy, VM pressure and exercise of often suck fsync/O_SYNC codepatheses, with only a benefit if you have multiple VMs actually using the same dataset with the same COW backing image. Using WCE = 0 or 1 is a tradeoff of safety vs performance. WCE=0 performance generally sucks badly, but offers data integrity for older or non-PCish host operating systems.
