Random write result differences between RAID device and XFS

[Dave Chinner]

On Fri, Jan 29, 2016 at 11:53:35AM +0100, Christian Affolter wrote:
> Hi everyone,
> 
> I'm trying to understand the differences of some bandwidth and IOPs test
> results I see while running a random-write full-stripe-width aligned fio
> test (using libaio with direct IO) on a hardware RAID 6 raw device
> versus on the same device with the XFS file system on top of it.
> 
> On the raw device I get:
> write: io=24828MB, bw=423132KB/s, iops=137, runt= 60085msec
> 
> With XFS on top of it:
> write: io=14658MB, bw=249407KB/s, iops=81, runt= 60182msec

Now repeat with a file that is contiguously allocated before you
start. And also perhaps with the "swalloc" mount option.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com