On Tue, Jan 15, 2013 at 09:09:38AM -0800, Charles Polisher wrote: > On Tue, Jan 15, 2013 at 11:55:07PM +1100, Peter Rabbitson wrote: > > On Tue, Jan 15, 2013 at 07:49:10AM -0500, Phil Turmel wrote: > > > You are neglecting each drive's need to skip over parity blocks. If the > > > array's chunk size is small, the drives won't have to seek, just wait > > > for the platter spin. Larger chunks might need a seek. > > > > > Either way, you > > > won't get better than (single drive rate) * (n-2) where "n" is the > > > number of drives in your array. (Large sequential reads.) > > > > This can't be right. As far as I know the md layer is smarter than that, and > > includes various anticipatory codepaths specifically to leverage multiple > > drives in this fashion. Fwiw raid5 does give me the near-expected speed > > (n * single drive). > > Happen to be working with comparative benchmarks looking for > relative throughput, varying the number of active drives in the > array and the RAID level. Clearly in this data RAID6 sequential > writes are bottlenecked by the 2 parity stripes. RAID6 setup > increases from 2 non-parity drives in the 4 drive configuration > to 6 non-parity drives in the 8 drive configuration, so one > might hope for 3x advantage. Yet the data show an advantage of > only 1.83 for reads. My guess is the need to read the parity > stripes is again a limiting factor. Next benchmark will vary > stripe and stride. > > Advantage Advantage > vs 4 drives vs RAID0 > Config Drives Seq write Seq read Write Read Write Read > ------ ------ ---------- ---------- ----- ----- ---- ---- > RAID0 4 8.1MB/sec 9.3MB/sec 1.00 1.00 1.00 1.00 > RAID0 8 16.8MB/sec 15.0MB/sec 2.07 1.61 1.00 1.00 > > RAID1 4 2.1MB/sec 3.6MB/sec 1.00 1.00 0.25 0.38 > RAID1 8 1.6MB/sec 3.6MB/sec 0.76 1.00 0.09 0.24 > > RAID5 4 16.8MB/sec 9.1MB/sec 1.00 1.00 2.07 0.97 > RAID5 8 17.2MB/sec 14.9MB/sec 1.02 1.63 2.12 1.60 > > RAID6 4 12.6MB/sec 7.9MB/sec 1.00 1.00 1.55 0.84 > RAID6 8 14.4MB/sec 14.5MB/sec 1.63 1.83 1.77 1.55 > > RAID10 4 4.0MB/sec 7.3MB/sec 1.00 1.00 0.49 0.78 > RAID10 8 6.3MB/sec 13.4MB/sec 1.57 1.83 0.37 0.89 > > Yes, these drives are *really* slow (Connor CP 30548). > The math doesn't change. > -- > Charles What layout are you using for RAID10? Is it Linux MD RAID10? 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
