On 2/17/2012 1:03 PM, Peter Grandi wrote: > As I have repeated many many many times to you in past XFS > discussions, and please take note, stripe alignment matters ONLY > AND SOLELY IF READ-MODIFY-WRITE is involved, and RADI10 never > requires read-modify-write. A wise Jedi once contradicted himself when he advised me to "never speak in absolute terms". A wiser Jedi would have said "don't speak in absolute terms". [...] > Disclaimer: using stripe alignment even when it is not required > may help a bit with scheduling, it being slightly akin to a > larger block size, but not quite, but that is a secondary > effect. And the inevitable future contradiction is the reason. The wisest of Jedi once told me "do not leave performance on the table". Whether you consider this "scheduling" effect, or others, of write alignment on non RMW devices to be secondary, it does have positive performance implications, and should thus not be left on the table. More importantly, and I could be mistaken, but IIRC, even absent an underlying RMW block device, XFS journal writes benefit from alignment due to a resulting lower ratio of write barriers issued to blocks written, due to the larger stripe width write out. And as we all know (or should), write barriers can murder performance, especially for metadata heavy workloads, as each barrier operation typically flushes all the drives' caches. Most mdraid users probably don't run BBWC RAID cards in JBOD mode to avoid barriers, though I know of a few who do. So XFS write alignment on mdraid should be an issue for most everyone using XFS, regardless of whether their array is parity RMW or not. Both unaligned RMW and write barriers are performance killers. Which one has more blood on its hands I can't say. I've never done such testing nor come across a related paper. -- Stan -- 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
