Hello, On 07/28/2010 10:50 AM, Christoph Hellwig wrote: > On Wed, Jul 28, 2010 at 10:40:30AM +0200, Tejun Heo wrote: >> The barrier machinery can be easily changed to drop the DRAIN and >> ordering stages, > > Maybe you're smarted than me, but so far I had real trouble with that. It's more likely that I was just blowing out hot air as I haven't looked at the code for a couple of years now. So, well, yeah, let's drop "easily" from the original sentence. :-) > The problem is that we actually still need the drain colouring to > keep out other "barrier" requests given that we have the state for > the pre- and post- flush requests in struct request. This and dealing > is where I'm still struggling with my the even more relaxed barriers > I had been working on for a while. They work perfectly on devices > supporting the FUA bit and nothing inbetween. > >> so all we need to do is an interface for the >> filesystem to tell the barrier implementation that it will take care >> of ordering itself and barriers (a bit of misnomer but well it isn't >> too bad) can be handled as FUA writes which get executed after all >> previous commansd are committed to NV media. On write-through device >> w/ FUA support, it will simply become a FUA write. > > If the device is write through there is not need for the FUA bit to > start with. Oh, right. >> On a device w/ >> write back cache and w/o FUA support, it will become flush, write, >> flush sequence. On a device inbetween, flush, FUA write. Would that >> be enough for filesystems? If so, the transition would be pretty >> painless, md already splits barriers correctly and the modification is >> confined to barrier implementation itself and filesystem which want to >> use more relaxed ordering. > > The above is a good start. But at least for XFS we'll eventually > want writes without the pre flush, too. We'll only need the pre-flush > for a specific class of log writes (when we had an extending write or > need to push the log tail), otherwise plain FUA semantics are enough. > Just going for the pre-flush / FUA semantics as a start has the > big advantage of making the transition a lot simpler, though. I see. It probably would be good to have ordering requirements carried in the bio / request, so that filesystems can mix and match barriers of different strengths as necesasry. As you seem to be already working on it, are you interested in pursuing that direction? Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
