IBM SG24-5360-00 manual VSE/ESA Support for the RVA, What Is IXFP/SnapShot for VSE/ESA?

1.2 VSE/ESA Support for the RVA

The RVA has been supported by VSE/ESA since its introduction. Because the RVA presents itself as logical 3380 or 3390 direct access storage devices (DASD) attached to a logical 3990 Model 3 storage control, releases of VSE/ESA supporting this logical environment have functioned with the RVA. However, until now, VSE/ESA has not provided SnapShot, deleted data space release (DDSR) or capacity reporting natively. It has depended on VM/ESA ′s IXFP and SnapShot to provide that benefit to VSE/ESA guests.

1.2.1 What Is IXFP/SnapShot for VSE/ESA?

IXFP/SnapShot for VSE/ESA is a feature of VSE Central Functions in conjunction with VSE/ESA Version 2, Release 3.1. It provides SnapShot, DDSR, and capacity reporting support for the RVA. OS/390 and VM/ESA provide two distinct productsÐIXFP and SnapShotÐfor supporting the RVA, whereas VSE/ESA has integrated many of the functions provided by these products into a single feature that implements the support in an Attention Routine command.

1.2.2 What Is SnapShot?

SnapShot, one of the three functions in IXFP/SnapShot for VSE/ESA, enables you to produce almost instantaneous copies of non-VSAM data sets, volumes, and data within CYLINDER ranges.

Note: Although not officially supported, VSAM data sets can be indirectly copied through techniques we discuss in Appendix C, ªVSE/VSAM Considerationsº on page 59.

The speed in copying is attained by exploiting the RVA′s virtual disk architecture. Snapshot produces copies without data movement. We call making a copy with SnapShot a snap. The result of a SnapShot is also called a snap.

Conventional methods of copying data on DASD consist of making a physical copy of the data on either DASD or tape. Host processors, channels, tape, and DASD controllers are involved in these conventional copy processes. Copying may take a long time, depending on available system resources.

In the RVA′s virtual disk architecture, a functional device is represented by a certain number of pointers in the FTD. Every used track has a pointer in the FTD to its back-end data. You snap a copy of the data by copying its FTD pointers. As you can imagine, snapping is a very fast process that takes seconds rather than minutes or hours. No data movement takes place, and no additional

back-end physical space is used. Both FTD pointers, the original and the copy, point to the same physical data location (see Figure 2 on page 4). Notice too that the TNT entry now has a value of 2, which indicates that the data is in use by two logical volumes.

Chapter 1. The IBM RAMAC Virtual Array 3