Your concern is justified.
The question is....real memory vs CPU.
You shouldn't have much of an I/O bottleneck with your caching
controller, assumming you have ficon or better channel speeds.
But if your read I/O is satisfied from MDC, you won't go thru the I/O
boundry which is a saving in CPU time.
So the question becomes can you allocate sufficient real memory for MDC
in order to have a sufficiently high MDC read hit ratio, to have a real
savings in CPU? Or do you care about a few percent savings in CPU?
If you are tight in main memory, it may be better to eliminate MDC and
use the memory to reduce paging.
If you are tight in CPU, then the CPU savings may be worth it.
An old rule of thumb was caching closer to the application is better
than caching farther away from the application. But that is only if the
memory for caching was of equal sizes. I would rather have 6 GB
controller cache, then 2 MB for VSAM buffers.
Anyway, I would experiment with MDC cache. If you can't get a high hit
ratio, say 95% or better, I would turn it off. But there is always
"that application" that may benefit greatly, for a short period of time,
by the use of MDC.
Tom Duerbusch
THD Consulting
>>> [EMAIL PROTECTED] 7/11/2006 1:27 PM >>>
Hello Everyone,
I have found some time here to re-evaluate some parameters.
We have a large amount of Cache (6 gig) on the EMC box. The
EMC
is doing lots of
caching.
I am wondering about the overhead of the dual caching and the
benefits.
It seems to me that having MDC on for the system is just overhead and
dual caching.
z/VM side
q cache 740
0740 CACHE 0 available for subsystem
0740 CACHE 1 available for subsystem
06324150K Bytes configured
06324150K Bytes available
00000000K Bytes offline
00000000K Bytes pinned
0740 CACHE activated for device
VSE/ESA side
cache subsys=740,status
AR 0015 SUBSYSTEM CACHING STATUS: ACTIVE
AR 0015 CACHE FAST WRITE: ACTIVE
AR 0015 CACHE STORAGE: CONFIG. ....... 6324150K
AR 0015 CACHE STORAGE: AVAIL. ....... 6324150K
AR 0015 NVS STATUS: AVAILABLE
AR 0015 NVS STORAGE: CONFIG. ....... 196608K
AR 0015 1I40I READY
cache subsys=740,report
AR 0015 3990-E9 SUBSYSTEM COUNTERS REPORT
AR 0015 VOLUME 'RAM040' DEVICE ID=X'00'
AR 0015 CHANNEL OPERATIONS
AR 0015 <----SEARCH/READ---->
<-------------WRITE------------>
AR 0015 <----SEARCH/READ---->
<-------------WRITE------------>
AR 0015 TOTAL CACHE-READ TOTAL CACHE-WRITE
DASD-FAST
AR 0015 REQUESTS
AR 0015 NORMAL 837170781 824709019 7467393 7463857
7467393
AR 0015 SEQUENTIAL 13620747 13148843 168445 168286
168445
AR 0015 CACHE FAST WRT 0 0 0 0
N/A
AR 0015
AR 0015 TOTALS 850791528 837857862 7635838 7632143
7635838
AR 0015
AR 0015 REQUESTS
AR 0015 INHIBIT CACHE LOADING 0
AR 0015 BYPASS CACHE 31
AR 0015
AR 0015 DATA TRANSFERS DASD->CACHE CACHE->DASD
AR 0015 NORMAL 9571687 762405
AR 0015 SEQUENTIAL 1600428 N/A
AR 0015 1I40I READY
Ed Martin
Aultman Health Foundation
330-588-4723
[EMAIL PROTECTED]
ext. 40441
