Re: priocscan vs fcfs

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Thu, Dec 03, 2015 at 07:43:10PM +0100, Petar Bogdanovic wrote:
> On Wed, Dec 02, 2015 at 08:23:28PM +0000, Michael van Elst wrote:
> > 
> > That's probably why setting the queues all to fcfs is the best
> > for you.
> 
> Not as dramatic as Emile's numbers but significantly higher read
> throughput:

What strategy are you using to sort inside RAIDframe?  This is a property
of the RAID set; you can see it with raidctl I believe for autoconfigured
sets.

I wouldn't be terribly surprised to see bad interactions between
priocscan and RAIDframe's scan or cscan strategies.  What about the "fifo"
strategy?

It's important, though, to understand that there are *very* few pure bulk
read applications in this world (except single-stream video) and very
few pure bulk write applications except database logs.  That means that
single-stream pure read or write tests are really pretty awful predictors
of disk performance for real workloads.

The "priocscan" strategy, in particular, limits pure read/pure write
performance *by design* in order to achieve lower latency under real
world mixed workloads.  I would not be so quick to discard it -- though I
would not use it on something like an SSD, and I am skeptical it would
perform well when stacked on one of RAIDframe's elevator-sort variants.

Thor