On Sun, Dec 14, 2008 at 10:02:05PM +0000, Peter Grandi wrote: > [ ... ] > > > But - as far as I understood - the filesystem doesn't have to > > wait for barriers to complete, but could continue issuing IO > > requests happily. A barrier only means, any request prior to > > that have to land before and any after it after it. > > > It doesn't mean that the barrier has to land immediately and > > the filesystem has to wait for this. At least that always was > > the whole point of barriers for me. If thats not the case I > > misunderstood the purpose of barriers to the maximum extent > > possible. > > Unfortunately that seems the case. > > The purpose of barriers is to guarantee that relevant data is > known to be on persistent storage (kind of hardware 'fsync'). > > In effect write barrier means "tell me when relevant data is on > persistent storage", or less precisely "flush/sync writes now > and tell me when it is done". Properties as to ordering are just > a side effect. No, that is incorrect. Barriers provide strong ordering semantics. I/Os issued before the barrier must be completed before the barrier I/O, and I/Os issued after the barrier write must not be started before the barrier write completes. The elevators are not allowed to re-оrder I/Os around barriers. This is all documented in Documentation/block/barrier.txt. Please read it because most of what you are saying appears to be based on incorrect assumptions about what barriers do. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
