z/VM: CP Planning and Administration - z/VM - IBM

Chapter 19. Expanded Storage Planning and AdministrationOverviewThe Expanded Storage facility is an optional feature of certain processor models. Itcan be:v Dedicated to CPv Dedicated to one or more virtual machines for use by an operating systemv Partitioned for use between CP and one or more virtual machines.This chapter tells you what Expanded Storage is and how to:v Attach Expanded Storage to a virtual machinev Map Expanded Storagev Handle fragmentation of Expanded Storagev Allocate Expanded Storage to CPv Allocate retained Expanded Storagev Use Expanded Storage as a minidisk cache.Expanded Storage acts as another level of real storage. It allows for the rapidtransfer of 4 KB pages between itself and real storage. As real storage is dividedinto 4 KB page frames, Expanded Storage is divided into 4 KB units called blocks.It is available in increments whose size varies with the model of the processor.You can partition Expanded Storage. Part of it may be attached to one or morevirtual machines. Another part, the CP partition, is used for system paging and thecaching of certain kinds of data. For paging, CP uses blocks available in the pagingportion of the CP partition in preference to DASD slots.To retain some or all of Expanded Storage exclusively for CP use, the class Boperator can enter the RETAIN XSTORE command. The command assigns thespecified amount of storage to the CP partition. The CP partition size limits theamount of Expanded Storage available for virtual machine partitions.All or part of Expanded Storage can be retained for CP’s exclusive use. This CPpartition can occupy multiple discontinuous areas of Expanded Storage. All or partof the remainder can be dedicated to virtual machines.If there is a request for additional Expanded Storage to be retained by CP and notenough contiguous Expanded Storage is available, the request can be satisfied byretaining multiple discontinuous areas of Expanded Storage. If a request to retainExpanded Storage for CP use is for a larger amount than is available because it isin use by one or more virtual machines, CP retains the amount of ExpandedStorage currently available. It also issues a pending request for the remainder. Thepending request is satisfied when enough Expanded Storage is detached from oneor more virtual machines.z/VM treats Expanded Storage as another level of real storage. All paging goes toExpanded Storage if possible. Old pages from Expanded Storage are migrated toDASD when paging use exceeds a high threshold. For the most part, direct pagingto DASD occurs only if Expanded Storage is totally filled and migration cannotmake room in Expanded Storage quickly enough to keep up with the pagingdemand.© Copyright IBM Corp. 1991, 2004 591