realtime section bugs still around

[Stan Hoeppner]

On 8/2/2012 5:39 AM, Stan Hoeppner wrote:

> We already gave you the biggest cause of your latency, which is garbage
> collection/wear leveling.  You can't see inside the SSDs, but you can
> see the latency jump with either top (%wa) or iostat (await,
> milliseconds).  Run
> 
> iostat -x -d 1 20
> 
> and you get 20 reports 1 second apart.  1s is minimum granularity.  This
> should clearly show the latency spikes caused by the SSDs.  Maybe even
> execute it for 60 seconds and pipe to a file.

The above assumes Linux can see the individual devices.  I've never used
Intel's fakeraid.  If its driver presents a single device to the kernel
instead of both SSD devices, iostat won't show which SSD's garbage
collection is kicking in and/or when.  It would be most beneficial if
you could see the iostat data for both SSD devices as it would tell you
exactly when each drive's GC/leveling kicks in.  If the Intel fakeraid
doesn't allow you to see both devices, you'll need to switch to md/RAID.
 I'm sure that will be problematic as you're very likely booting from
the Intel RAIDed SSD device.

-- 
Stan