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?
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