On Wednesday February 6, davidsen@xxxxxxx wrote: > Keld Jørn Simonsen wrote: > > Hi > > > > I am looking at revising our howto. I see a number of places where a > > chunk size of 32 kiB is recommended, and even recommendations on > > maybe using sizes of 4 kiB. > > > > > Depending on the raid level, a write smaller than the chunk size causes > the chunk to be read, altered, and rewritten, vs. just written if the > write is a multiple of chunk size. Many filesystems by default use a 4k > page size and writes. I believe this is the reasoning behind the > suggestion of small chunk sizes. Sequential vs. random and raid level > are important here, there's no one size to work best in all cases. Not in md/raid. RAID4/5/6 will do a read-modify-write if you are writing less than one *page*, but then they often to read-modify-write anyway for parity updates. No level will every read a whole chunk just because it is a chunk. To answer the original question: The only way to be sure is to test your hardware with your workload with different chunk sizes. But I suspect that around 256K is good on current hardware. NeilBrown - 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
