>>>>> "Markus" == Markus Stockhausen <stockhausen@xxxxxxxxxxx> writes: >> Von: John Stoffel [john@xxxxxxxxxxx] >> Gesendet: Sonntag, 24. August 2014 14:46 >> An: Markus Stockhausen >> Cc: linux-raid@xxxxxxxxxxxxxxx >> Betreff: Re: [PATCH v3 5/6] md/raid5: activate raid6 rmw feature >> ... >> Also, how does the patch look when you have a simple 4 disk RAID6 array? >> I would hope that since the parity overhead is much higher, it would show >> more improvement as well. Markus> Maybe I did not explain exactly what this patch is for. To boil it down to four Markus> short and simple key facts. Markus> - The current kernel implementation of RAID6 is very I/O expensive for small Markus> changes. We always need to read all unchanged blocks of the same stripe, Markus> recalulate the parity and write the new data block plus two parities. So a single Markus> block change will produce n (number of disks) I/Os. The source code calls it Markus> RCU - reconstruct write. Markus> - With the patch we will only read the old data of the to be changed block & the Markus> parities, recalculate the parities with the new data and rewrite everthing back. A Markus> single block change will only have 6 I/Os regardless of the number of disks. It Markus> is called RMW - read modify write. Markus> - This patch will only kick in if you have 7 or more disks in a RAID6. And ONLY Markus> if I/Os RMW < I/Os RCU. This should answer your question above. Markus> - The more disks we have and the smaller the changed data is the more benefit Markus> we will get from that patch. This is a great summary of why this change could/should be in MD. I really think this should be part of the commit and/or docs. Also, showing that the results are an improvement in terms of IOPS would be awesome. So have you run something like that postmark test, which writes lots of small files into a filesystem and seen how that works with your patchset? I don't doubt that there's an improvement, I'm just wondering if it's enough. You just haven't justified it properly to me (though I'm not important, Neil is the decider here...) and I do appreciate your responses, since they have clarified things quite alot. John -- 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
