Unfortunately we are emulating the geometry of the last generation of ECDK devices (3390's) by creating virtual volumes within an array of SCSI disks.
In its day the model 9 was a bulk data storage model with poor performance characteristics. The performance of virtual 3390 volume however is a function of the real hardware used to build the SCSI array. Some systems programmers still persist in defining only 3390-3 volumes concerned over the performance issues associated with real 3390-9's. This results in a larger number of volumes to manage than necessary. Given today's multi-terabyte DASD farms, the number of logical volumes becomes staggering. It is however true that I/O is queued at the device level and there may be situations where more emulated volumes are better. However one can do no more transfers than the number of upper interfaces available (number of channel connections to the subsystem). In developing a capacity management system for a client last year, reporting for DASD services times only listed volumes whose services times exceeded 8 milliseconds. We have come a long way since the 1st cached controllers in the mid-80's. 3390's were state of the art disk drives 10 YEARS ago. Even if you were given the required 3990 controllers and some 3390 drives, it would still be cheaper to purchase a new Shark, EMC, etc. storage subsystem due to the cost of maintenance and the cost of power to spin them and to cool them. That is to say that real 3390's are for all practical purposes no longer in use. I have not been in a data center in the last 6 years that had any 3390's install. Maybe I have been lucky or oblivious to their presence. Has anyone heard of any IBM plans to provide non-ECKD DASD support for zOS? We can't go on emulating 33X0 volumes much longer. Don Stubbs -----Original Message----- From: Linux on 390 Port [mailto:[EMAIL PROTECTED]]On Behalf Of Peter Webb, Toronto Transit Commission Sent: Thursday, April 18, 2002 11:07 AM To: [EMAIL PROTECTED] Subject: Re: How to pre-allocate a 4GB File? > I'm sadly out of touch with mainframe hardware; it seems strange to me > I can get 120 Gbyte disks for my PC and you folk are using 2.3 Gbyte > drives on your mainframes. I know the theoretical limit is larger than > that, and was back when S/360 was announced. > > One advantage to the limited disk size is that since you can only have one I/O active to a disk volume at one time, spreading data over many smaller volumes allows more I/Os to be active at the same time. This can dramatically improve system I/O throughput. IBM even warns that using 3390-9s can degrade performance, think what using a 3390-120Gigabyte would do! In the PC world, I believe you can see the same sort of effect by using RAID0 across multiple volumes, versus one large volume.
