Il 20/02/2013 14:46, Kevin Wolf ha scritto: >> > Thanks for the fast reply. Can you please elaborate on that (or post a >> > link to an explanation): What exactly does the backend have to provide >> > that for example cache=writeback can be used? Is there a way to test for >> > it? > The problem is about cache coherency. Local files work just fine with > cache=writeback, but if you migrate to a different host, you get the > problem that the destination kernel may cache some parts of the image > while the source is still writing to it. When you do the actual switch > to the destination host and the backend doesn't take care of > invalidating that cache, you might (partially) read outdated data.
Also, note that some backends (iSCSI and NBD for example) will never use a cache on the host, so they can be used with the default cache=writeback mode. However, this is simply because writeback is mapped to none, and writethrough to directsync. If you used iSCSI or NBD through the kernel, you would need cache=none. Paolo
