>>> On Sun, 23 Dec 2007 08:26:55 -0600, "Jon Nelson" >>> <jnelson-linux-raid@xxxxxxxxxxx> said: > I've found in some tests that raid10,f2 gives me the best I/O > of any raid5 or raid10 format. Mostly, depending on type of workload. Anyhow in general most forms of RAID10 are cool, and handle disk losses better and so on. > However, the performance of raid10,o2 and raid10,n2 in > degraded mode is nearly identical to the non-degraded mode > performance (for me, this hovers around 100MB/s). You don't say how many drives you got, but may suggest that your array transfers are limited by the PCI host bus speed. > raid10,f2 has degraded mode performance, writing, that is > indistinguishable from it's non-degraded mode performance > It's the raid10,f2 *read* performance in degraded mode that is > strange - I get almost exactly 50% of the non-degraded mode > read performance. Why is that? Well, the best description I found of the odd Linux RAID10 modes is here: http://en.Wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10 The key here is: "The driver also supports a "far" layout where all the drives are divided into f sections." Now when there are two sections as in 'f2', each block will be written to a block in the first half of the first disk and to the second half of the "next" disk. Consider this layout for the first 4 blocks on 2x2 layout compared to the standard layout: DISK DISK A B C D A B C D 1 2 3 4 1 1 2 2 . . . . 3 3 4 4 . . . . . . . . ------- 4 1 2 3 . . . . . . . . . . . . This means that with the far layout one can read blocks 1,2,3,4 at the same speed as a RAID0 on the outer cylinders of each disk; but if one of the disks fails, the mirror blocks have to be read from the inner cylinders of the next disk, which are usually a lot slower than the outer ones. Now, there is a very interesting detail here: one idea about getting a fast array is to take make it out of large high density drives and just use the outer cylinders of each drive, thus at the same time having a much smaller range of arm travel and higher transfer rates. The 'f2' layout means that (until a drive fails) for all reads and for "short" writes MD is effectively using just the outer half of each drive, *as well as* what is effectively a RAID0 layout. Note that the sustained writing speed of 'f2' is going to be same *across the whole capacity* of the RAID. While the sustained write speed of a 'n2' layout will be higher at the beginning and slower at the end just like for a single disk. Interesting, I hadn't realized that, even if I am keenly aware of the non uniform speeds of disks across cylinders. - 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
