Disclaimer: I am not a performance expert, so take this with a large
grain of salt.
I agree with what Ron wrote: That synchronously replicated disk I/O
write response times are longer than those for volumes that are not
replicated is not surprising. For basic PPRC it will be higher to start
with, and distance makes things worse.
For 30km separation, I get ~.2ms round trip time just to get there and
back at the speed of light. Multiply that by the reciprocal of the
fiber's velocity factor. Let's call that .7 (I don't know the actual
number) which makes it about .3ms per PPRC exchange. I don't know how
many exchanges it takes to replicate something over PPRC, but you'd have
to multiply the .3ms by that number and add data transfer time to see
the effect of distance. If anything else affects your PPRC replication
traffic, these numbers can only go up.
You might want to see whether HyperWrite, which starts I/O to both disk
subsystems at more or less the same time, would help you enough to
consider using it. There's an article about it here:
http://ibmsystemsmag.com/mainframe/administrator/db2/zhyperwrite-zip/
It will mostly eliminate the initial PPRC write delays, which might be a
significant chunk of your replicated disk I/O response times. But
nothing can help you with the distance-imposed latency. We know how to
slow light down, but speeding it up is "more difficult." This is one
reason some people opt for a nonzero RPO when meaningful distances are
involved.
All the above are "back of a napkin" numbers. If I've got them wrong, I
am sure someone will jump in. I think the basic problems, on the other
hand, are well-understood. If you double-check the assumptions and math
and the result explains what you are seeing within reasonable spitting
distance, then there probably won't be much you can do (other than
perhaps HyperWrite).
On the other hand, if the latency due to distance does not explain what
you are seeing, then (as Ron pointed out) there are a number of things
you can check on (about which I personally know little or nothing).
Tommy Tsui wrote:
Hi,
The distance is around 30km, do you know any settings on sysplex
environment such as GRS and JES2 checkpoint need to aware?
Direct DASD via San switch to Dr site , 2GBPS interface , we check with
vendor, they didn't find any problem on San switch or DASD, I suspect the
system settings
<snip>
--
John Eells
IBM Poughkeepsie
[email protected]
----------------------------------------------------------------------
For IBM-MAIN subscribe / signoff / archive access instructions,
send email to [email protected] with the message: INFO IBM-MAIN
