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Subject: Re: [virtio-dev] Re: [virtio-comment] Problems with VIRTIO-4 and writeback only disks
On Tue, Sep 17, 2013 at 03:03:47PM +0930, Rusty Russell wrote: > James Bottomley <firstname.lastname@example.org> writes: > > On Mon, 2013-09-16 at 16:50 +0930, Rusty Russell wrote: > >> I think we should do the reverse: > >> 1) If you negotiate VIRTIO_BLK_F_WCE, you need to flush (ie. it's > >> writeback). Otherwise it's write-through. > >> 2) If (and only if) you negotiate VIRTIO_BLK_F_WCE, you may also negotiate > >> VIRTIO_BLK_F_CONFIG_WCE, in which case you can toggle it. > >> > >> But I'm not convinced that #2 should exist at all. Does someone have a > >> real use case? > > > > There is no use case in Linux. We provide knobs to toggle the cache > > type in the SCSI disk class, but the kernel doesn't do it by default, it > > just runs with the cache type because the toggle can be unreliable. > > OK. How about this as a strawman: > > 1) Kill VIRTIO_BLK_F_CONFIG_WCE as a bad idea. You either negotiate > WCE or not during device setup; no toggling. It's not really useful as guest-controlled, I agree. But I have this vague idea: - host wants to enable WCE on backend - host notifies guest - guest enables WCE - host can now enable WCE on backend > 2) If device offers VIRTIO_BLK_F_WCE, and driver negotiates it: > - Completed writes should be persistent if guest crashes. > - No ordering guarantees are given except where flush command sent. > - Flush SHOULD push all preceeding writes to permanent storage. > (in the RFC SHOULD sense, which is weaker than MUST). We can also document that it's legal for device to make the feature required (using the new handshake). > 3) If device does not offer VIRTIO_BLK_F_WCE, or driver doesn't > negotiate it: > - Completed writes should be persistent if guest crashes. > - No flush commands are supported. > - No guarantee about writes hitting permanent storage. > > This pretty neatly divides it into complex and simple cases. If you > want more fine-grained, you know where to find virtio-scsi... > > Cheers, > Rusty.