On Friday, April 15, 2011 07:24 PM, Benjamin Franz wrote: > On 04/14/2011 09:00 PM, Christopher Chan wrote: >> >> Wanna try that again with 64MB of cache only and tell us whether there >> is a difference in performance? >> >> There is a reason why 3ware 85xx cards were complete rubbish when used >> for raid5 and which led to the 95xx/96xx series. >> _ > > I don't happen to have any systems I can test with the 1.5TB drives > without controller cache right now, but I have a system with some old > 500GB drives (which are about half as fast as the 1.5TB drives in > individual sustained I/O throughput) attached directly to onboard SATA > ports in a 8 x RAID6 with *no* controller cache at all. The machine has > 16GB of RAM and bonnie++ therefore used 32GB of data for the test. > > Version 1.96 ------Sequential Output------ --Sequential Input- > --Random- > Concurrency 1 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- > --Seeks-- > Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP > /sec %CP > pbox3 32160M 389 98 76709 22 91071 26 2209 95 264892 26 > 590.5 11 > Latency 24190us 1244ms 1580ms 60411us 69901us > 42586us > Version 1.96 ------Sequential Create------ --------Random > Create-------- > pbox3 -Create-- --Read--- -Delete-- -Create-- --Read--- > -Delete-- > files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP > /sec %CP > 16 10910 31 +++++ +++ +++++ +++ 29293 80 +++++ +++ > +++++ +++ > Latency 775us 610us 979us 740us 370us > 380us > > Given that the underlaying drives are effectively something like half as > fast as the drives in the other test, the results are quite comparable. Woohoo, next we will be seeing md raid6 also giving comparable results if that is the case. I am not the only person on this list that thinks cache is king for raid5/6 on hardware raid boards and the using hardware raid + bbu cache for better performance one of the two reasons why we don't do md raid5/6. > > Cache doesn't make a lot of difference when you quickly write a lot more > data than the cache can hold. The limiting factor becomes the slowest > component - usually the drives themselves. Cache isn't magic performance > pixie dust. It helps in certain use cases and is nearly irrelevant in > others. > Yeah, you are right - but cache is primarily to buffer the writes for performance. Why else go through the expense of getting bbu cache? So what happens when you tweak bonnie a bit?