On Fri, Sep 01, 2017 at 06:39:09PM +0000, Richard Wareing wrote: > Thanks for the quick feedback Dave! My comments are in-line below. > > > > On Aug 31, 2017, at 9:31 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > > > Hi Richard, > > > > On Thu, Aug 31, 2017 at 06:00:21PM -0700, Richard Wareing wrote: ... > >> add > >> support for the more sophisticated AG based block allocator to RT > >> (bitmapped version works well for us, but multi-threaded use-cases > >> might not do as well). > > > > That's a great big can of worms - not sure we want to open it. The > > simplicity of the rt allocator is one of it's major benefits to > > workloads that require deterministic allocation behaviour... > > Agreed, I took a quick look at what it might take and came to a similar conclusion, but I can dream :). > Just a side point based on the discussion so far... I kind of get the impression that the primary reason for using realtime support here is for the simple fact that it's a separate physical device. That provides a basic mechanism to split files across fast and slow physical storage based on some up-front heuristic. The fact that the realtime feature uses a separate allocation algorithm is actually irrelevant (and possibly a problem in the future). Is that an accurate assessment? If so, it makes me wonder whether it's worth thinking about if there are ways to get the same behavior using traditional functionality. This ignores Dave's question about how much of the performance actually comes from simply separating out the log, but for example suppose we had a JBOD block device made up of a combination of spinning and solid state disks via device-mapper with the requirement that a boundary from fast -> slow and vice versa was always at something like a 100GB alignment. Then if you formatted that device with XFS using 100GB AGs (or whatever to make them line up), and could somehow tag each AG as "fast" or "slow" based on the known underlying device mapping, could you potentially get the same results by using the same heuristics to direct files to particular sets of AGs rather than between two physical devices? Obviously there are some differences like metadata being spread across the fast/slow devices (though I think we had such a thing as metadata only AGs), etc. I'm just handwaving here to try and better understand the goal. Brian > > > > Cheers, > > > > Dave. > > -- > > Dave Chinner > > david@xxxxxxxxxxxxx > > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
