Avi, cache=writeback can be faster than cache=none for the same reasons a disk cache speeds up access. As long as the I/O mix contains more asynchronous then synchronous writes it allows the host to do much more reordering, only limited by the cache size (which can be quite huge when using the host pagecache) and the amount of cache flushes coming from the host. If you have a fsync heavy workload or metadata operation with a filesystem like the current XFS you will get lots of cache flushes that make the use of the additional cache limits.
If you don't have a of lot of cache flushes, e.g. due to dumb applications that do not issue fsync, or even run ext3 in it's default mode never issues cache flushes the benefit will be enormous, but the data loss and possible corruption will be enormous. But even for something like btrfs that does provide data integrity but issues cache flushes fairly effeciently data=writeback may provide a quite nice speedup, especially if using multiple guest accessing the same spindle(s). But I wouldn't be surprised if IBM's exteme differences are indeed due to the extremly unsafe ext3 default behaviour. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
