> I'm going to guess that you're getting some small file writes, even if > they're just file system metadata updates scattered about the address > space. And little writes to many scattered areas means many 16MB * 8 = > full stripe writes every time for read, modify, write of a stripe. For > raid5 there's an optimization which I think is sector size, so for > just 512b of file system metadata changing, you'd get a 512b sector > write for the data (the fs metadata) and a 512b sector write for > parity. But I don't think there's any such optimization for raid6, so > maybe someone else can answer that question. AFAIK there is no difference between raid5 and raid6 in partial chunk update. I tried fio with small random writes (4kb) - no significant performance difference between arrays with chunk sizes 64Kib/512Kib/16Mib.
