[CentOS] 40TB File System Recommendations

Fri Apr 15 13:47:54 UTC 2011
Jerry Franz <jfranz at freerun.com>

On 04/15/2011 06:05 AM, Christopher Chan wrote:
> 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.

That *is* md RAID6. Sorry I didn't make that clear. I don't use anyone's 
hardware RAID6 right now because I haven't found a board so far that was 
as fast as using md. I get better performance from even a BBU backed 95X 
series 3ware board by using it to serve the drives as JBOD and then 
using md to do the actual raid.

> 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?

For smaller writes. When writes *do* fit in the cache you get a big 
bump. As I said: Helps some cases, not all cases. BBU backed cache helps 
if you have lots of small writes. Not so much if you are writing 
gigabytes of stuff more sequentially.

Benjamin Franz