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.
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.
--
Benjamin Franz