On Tue, Dec 09, 2008 at 10:41:47PM +0100, Manuel Bouyer wrote: > On Tue, Dec 09, 2008 at 01:30:10PM -0800, Jason Thorpe wrote: > > >But to enforce that using FUA, you have to set FUA on *every* write. > > > > The only thing you have to ensure is that journal is consistent and on- > > disk before the other metadata blobs. > > I don't think it's so simple. I think there's also metadata writes that > needs to happen *before* journal writes. The only place I see FUA on non-journal writes would matter would be to make it easier to roll the journal. > > So use FUA and wait for the > > journal writes to complete BEFORE ISSUING the other writes. > > OK, so you use software barrier, not the ones provided by disks > (this is what was not clear). > But I think with this we'll end up issuing mostly FUA writes, because > ordering constraint are both way. So it won't be any better than > running simple queue tags with the write cache disabled (what we're > doing now). I think the advantage of FUA over write cache off is if we aren't journaling file data. If we are journaling everything, then write-cache off + simple tags will suffice. But my experience is to just journal fs metadata, so the difference is important. > > >And I do not understand the "only way" in your remark above. Under > > >the constraint "if WCE is set in the cache control page" I agree that > > >it is correct. But as far as I can tell, if WCE is *not* set, it is > > >not in fact the case that using FUA is the "only way" to ensure that > > >writes are committed to stable storage in order -- because the tag > > >ordering rules require ordered tags to complete, um, well, in-order, > > >and if WCE is not set, commands are not supposed to complete until > > >the bits are on oxide. > > > > > >I am sure I am misunderstanding something. What? > > > > Turning off the write cache makes things too bloody slow. > > With tagged queuing and simple queue tag it's not my experience. It is however the experience of others. Take care, Bill
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