Hi, > > OK. How fast are the Fujitsu disks, measured by a simple hdparm -t? Timing buffered disk reads: 270 MB in 3.02 seconds = 89.52 MB/sec > > OK, so the problem you reported earlier, that HW raid was faster than > raid10,f2 for writing, is gone? > Well actually not for sequetial output. Here are the most comparable figures: 4dk hw raid 10, 256k chunks versus 4dks md raid 10, 256k chunks per char sequential output = 69475 vs 69928 <------ this is similar block sequential output = 159649 vs 93985 <------ hw is much faster rewrite sequential output = 85914 vs 56930 <------ hw is much faster But on reading, md is faster: per char sequential input = 61622 vs 68669 <------ still comparable block sequential input = 221771 vs 356923 <------ md is way faster ransom seek = 1327.1 vs 1149.7 <------ that's a 15.4% difference > And you do a HW RAID10? Are you able to specify chunk size here? Yes, RAID10 is an option in Adaptec's bios, and chink size can be set up to 512k > > -sequential input varies greatly, the big winner being md-f2-256 setup > > with 356923K/sec, and the big loser md-near-64 setup with 34888K/sec > > (factor of 10 !) > > Both the chunk size, and the observation that raid10,n2 only reads from one > disk at a time, gives reasons to this. I already explained why raid10,f2 > would be faster than HW RAID10. > True, and quite impressive. > > - what seems the most relevant to me, random seeks are always better on > > software raid, by 10 to 20%, but I have no idea why. > > raid10.f2 woul only seek on half the disk, so that would diminish the > seek times. > Great. But in fact md raid 10 near layout (with 64k chunks, that might matter), gave me slitghly better results than l2 (1347.3 for near versus 1327.1 for far) > > - and running two bonnie++ in parallel on two 4 disks arrays gives > > better iops than 6 disks arrays. > > I would run a combined 12 disk array raid10,f2 with adequate chunk size, > I think that would get the best performance for you. > I will try that. > > So I tend to think I'd better use md-f2-256 with 3 arrays of 4 disks and > > use tablespaces to make sure my requests are spread out on the 3 arrays. > > But this conclusion may suffer from many many flaws, the first one being > > my understanding of raid, fs and io :) > > > > So, any comment ? > > I would try to test it out, but I don't know if you can get a good > benchmark for database enquieries. That's the real problem for sure. I can throw in some huge queries, but kernel resources and postgresql.conf clearly will change things much more than raw disk io. That why I thought of running several bonnie++ tests in parallel, and add random seek results to simulate database reading... Thanks Franck -- 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
