Il 10/01/2013 16:25, Jamie Lokier ha scritto: >> > Perhaps it's a bug that the cache mode is not reset when the machine is >> > reset. I haven't checked that, but it would be a valid complaint. > The question is, is cache=writeback/cache=writethrough an initial > setting of guest-visible WCE that the guest is allowed to change, or > is cache=writeththrough a way of saying "don't have a write cache" > (which may or may not be reflected in the guest-visible disk id).
It used to be the latter (with reflection in the disk data), but now it is the former. > I couldn't tell from QEMU documentation which is intended. It would > be a bit silly if it means different things for different backend > storage. It means the same thing for IDE, SCSI and virtio-blk. Other backends, such as SD, do not even have flush, and are really slow with cache=writethrough because they write one sector at a time. For this reason they cannot really be used in a "safe" manner. > I have seen (obscure) guest code which toggled WCE to simulate FUA, That's quite useless, since WCE=1->WCE=0 is documented to cause a flush (and it does). Might as well send a real flush. > and there is plenty of advice out there saying to set WCE=0 for > certain kinds of databases because of its presumed crash safety. Even > very ancient guests on Linux and Windows can change WCE=0 with IDE and > SCSI. This is supported in QEMU. > So from a guest point of view, I think guest setting WCE=0 should mean > exactly the same as FUA every write, or flush after every write, until > guest setting WCE=1. Yes, that's indeed how it is implemented. Paolo
