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* Default qemu options for RBD are bad.
* Bad means that a) it uses a the slow emulated LSI controller b) it uses a the mode which caches with cached reads, but not without cached writes.
* Drive cache is qemu is controlled by the `cache` option (surprise). It can be <missing>, writethrough, writeback, none, unsafe, directsync. With RBD this option also affects rbd cache, which is the cache on the Ceph’s client library (librbd) side.
* But cache=unsafe doesn’t work with RBD, it still waits for write confirmations. And writethrough, <missing> and directsync are basically equivalent.
