EMC storage array tweaking

[Jeff Macfarland]

Peter Grandi wrote:
>>>> On Wed, 05 Mar 2008 13:08:53 -0600, Jeff Macfarland
> 
> jmacfarland> Having cache on or off of the physical disks
> jmacfarland> themselves wont make anything faster
> 
> You would be surprised :-). Enabling the ondisk cache helps a
> lot for sequential IO both because it is ondisk (latency) and
> how it is used by the drive firmware.
> 
> Anyhow, to see how much it matters, try sequential writes after
> disabling the cache with 'hdparm -W0' on a simple ATA/SATA disk
> directly attached to a Linux PC (note how the problem is still
> present but smaller with 'O_DIRECT').

Yes, but we aren't talking about a simple sata drive attached to your
linux PC. Clearly a write cache will improve write performance..

> 
> jmacfarland> except perhaps if you're filling the cache and a
> jmacfarland> forced flush is popping up.
> 
> For bulk sequential IO the CX300 cache does not matter because
> what you say is then what happens all the time: the cache is
> always full after a short while (it takes a few seconds to fill
> 1GB either reading or writing) and all the IO thereafter is just
> cache spills (triggered by the upper/lower thresholds you
> mention below).
>

And you know a forced flush is happening? Or is that what you think is
happening? (just a question- i dont know you implementation). If it is
then array performance, according to a couple EMC instructors and techs,
will be abysmal. (but i guess you know that)

Even if it took only seconds to write 1GB over the wire a forcing a
flush, how long do you think it takes for the array to fill up somewhere
between 8-32 MB of cache on a drive cache?


> jmacfarland> Check your cache settings on the array to make sure
> jmacfarland> write caching is on and you have the upper and
> jmacfarland> lower flush limits correctly.
> 
> I'll have a look anyhow (the CX300 has been setup apparently by
> EMC consultants, so anything could have happened, like a setup
> involving RAID3/4 :->).

So, you're not sure of the raid type? If there hasn't been any other
work to accommodate your particular io requirements (multiple dae's,
load balancing io, playing around with flush levels)..I highly doubt
you're going to achieve it simply with physical disk caching.

Long story short, I don't know how :-) But good luck. HOpefully youll
find what youre looking for.

> Anyhow, I am still interested in any suggestion on how to check
> and if necessary tweak the ondisk cache of the disks on a
> Dell/EMC CX300...
> 
>