On Tue, Aug 04, 2009 at 03:21:28PM -0700, David Rees wrote: > 2009/8/1 Keld Jørn Simonsen <keld@xxxxxxxx>: > > On Sat, Aug 01, 2009 at 08:13:45AM -0700, David Rees wrote: > >> No - you're getting 120 MB/s from one disk and 80MB/s from another. > >> How that would add up to 230MB/s defies logic... > > > > Why only 80 MB/ when reading? reading from both disks with raid10,f2 are done at the > > beginning of both disks, thus getting about 115 MB/s from both of them. > > > > reading in raid10,f2 is restricted to the faster half of the disk, by > > design. > > > > It is different when writing. there both halves, fast and slow, are > > used. > > As I mentioned earlier, I was having a hard time visualizing the data > layout. So here's a simple diagram that shows near/far layout and why > Keld was right - with a far layout, reads can be isolated to the fast > half of the disk. > > It also shows how sequential writes (or any other write that spans > multiple chunks) force the drives to seek half way across the disk for > each write. > > Near layout, 4 disks, 2 copies: > a b c d > 0 0 1 1 > 2 2 3 3 > 4 4 5 5 > 6 6 7 7 > > Far layout, 4 disks, 2 copies > a b c d > 0 1 2 3 > 4 5 6 7 > 7 0 1 2 > 3 4 5 6 No, it is rather: Far layout, 4 disks, 2 copies a b c d 0 1 2 3 4 5 6 7 1 0 3 2 5 4 7 6 Best regards keld -- 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
