On 10/17/2017 02:32 PM, Anthony Youngman wrote: > On 17/10/17 01:33, NeilBrown wrote: >> This patch has some good stuff, but >> >> Chunk Size, as per the Linux RAID wiki, is the smallest unit of data >> that can be written to the devices >> >> I wonder where the Linux RAID wiki say that. It is wrong. >> The smallest unit of data that can be written to the devices is the >> block size, which is hardware dependant and typically 512bytes or 4K. >> This is not the same as the chunk size. >> > https://raid.wiki.kernel.org/index.php/RAID_setup#Chunk_sizes > > I understand what it's saying - this is the smallest size passed down to > the disk, not the smallest size that the disk can write ... :-) > > This page has made it into the archaeology section so I don't plan to > update it. And in context, it's reasonably clear what it means. No, it is *wrong*. Writes in multiples of 4k and entirely within a chunk are passes as-is to the devices. For mirrors, all affected devices get a copy of the request. For parity raid, the 4k stripes corresponding to those 4k blocks will be pulled into the stripe cache for recalculation. Not whole chunk-size stripes. The stripe cache is multiples of 4k, not multiples of the chunk size! Writes smaller than 4k, or not aligned to 4k, will generate a read-modify-write cycle of the 4k block involved. Not the whole chunk. It is more accurate to say that a chunk may be the *largest* a request can be before it is split between devices. Phil -- 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
