Hi All, I appreciate the feedback but most of it seems around File System recommendations or to change to parity-less RAID, like RAID 10. In my tests, there is no file system; I am testing the raw block device as I want to establish best-numbers there before layering on the file system. -Tommy On Mon, May 30, 2011 at 6:08 AM, David Brown <david@xxxxxxxxxxxxxxx> wrote: > On 30/05/2011 13:57, John Robinson wrote: >> >> On 30/05/2011 12:20, David Brown wrote: >>> >>> (This is in addition to what Stan said about filesystems, etc.) >> >> [...] >>> >>> Try your measurements with a raid10,far setup. It costs more on data >>> space, but should, I think, be quite a bit faster. >> >> I'd also be interested in what performance is like with RAID60, e.g. 4 >> 6-drive RAID6 sets, combined into one RAID0. I suggest this arrangement >> because it gives slightly better data space (33% better than the RAID10 >> arrangement), better redundancy (if that's a consideration[1]), and >> would keep all your stripe widths in powers of two, e.g. 64K chunk on >> the RAID6s would give a 256K stripe width and end up with an overall >> stripe width of 1M at the RAID0. >> > > Power-of-two stripe widths may be better for xfs than non-power-of-two > widths - perhaps Stan can answer that (he seems to know lots about xfs on > raid). But you have to be careful when testing and benchmarking - with > power-of-two stripe widths, it's easy to get great 4 MB performance but > terrible 5 MB performance. > > > As for the redundancy of raid6 (or 60) vs. raid10, the redundancy is > different but not necessarily better, depending on your failure types and > requirements. raid6 will tolerate any two drives failing, while raid10 will > tolerate up to half the drives failing as long as you don't lose both halves > of a pair. Depending on the chances of a random disk failing, if you have > enough disks then the chances of two disks in a pair failing are less than > the chances of three disks in a raid6 setup failing. And raid10 suffers > much less from running in degraded mode than raid6, and recovery is faster > and less stressful. So which is "better" depends on the user. > > Of course, there is no question about the differences in space efficiency - > that's easy to calculate. > > For greater paranoia, you can always go for raid15 or even raid16... > >> You will probably always have relatively poor small write performance >> with any parity RAID for reasons both David and Stan already pointed >> out, though the above might be the least worst, if you see what I mean. >> >> You could also try 3 8-drive RAID6s or 2 12-drive RAID6s but you'd >> definitely have to be careful - as Stan says - with your filesystem >> configuration because of the stripe widths, and the bigger your parity >> RAIDs the worse your small write and degraded performance becomes. >> >> Cheers, >> >> John. >> >> [1] RAID6 lets you get away with sector errors while rebuilding after a >> disc failure. In addition, as it happens, setting up this arrangement >> with two drives on each controller for each of the RAID6s would mean you >> could tolerate a controller failure, albeit with horrible performance >> and you would have no redundancy left. You could configure smaller >> RAID6s or RAID10 to tolerate a controller failure too. >> > > -- > 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 > -- 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
