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Asynchronous Allocation Requests

IP.com Disclosure Number: IPCOM000044942D
Original Publication Date: 1983-Jan-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Duvall, KE: AUTHOR [+2]

Abstract

In a data library subsystem environment having a large number of data storage volumes, the hosts in a multihost environment sharing the mass storage system can asynchronously or synchronously select a given volume for space allocation. During preallocation screening, the selected volume may appear to have sufficient contiguous space to all of the host processes which will or may request allocation space from that volume. When a particular host has selected a volume for allocation, the volume identification is put in an allocation queue for an allocating space manager, such as DASDSM (Direct-Access Storage Device Space Manager), to allocate space for the request. During the queue waiting time, another host may in fact allocate space on the selected volume.

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Asynchronous Allocation Requests

In a data library subsystem environment having a large number of data storage volumes, the hosts in a multihost environment sharing the mass storage system can asynchronously or synchronously select a given volume for space allocation. During preallocation screening, the selected volume may appear to have sufficient contiguous space to all of the host processes which will or may request allocation space from that volume. When a particular host has selected a volume for allocation, the volume identification is put in an allocation queue for an allocating space manager, such as DASDSM (Direct-Access Storage Device Space Manager), to allocate space for the request. During the queue waiting time, another host may in fact allocate space on the selected volume. As a result of the asynchronous intermediate allocation, the selected volume no longer may have sufficient contiguous space for the first host's space request. Each host will examine the space available, though perhaps fragmented (discontinuous) as being logically contiguous. Allocation then proceeds to allocate space contiguously when possible, but does not fail to allocate non-contiguously when necessary. In this manner, an extremely high percentage of allocation requests will be successful.

Multiple hosts share a large number of storage volumes. Storage volumes can be real storage volumes on a number of direct-access storage devices (DASDs). Such storage volumes may also be virtual volumes in an automated storage hierarchy employing a large data buffer coupled to a large backing store, such as an automated tape library. Each of the host processors (not shown) can activate a common program of instructions for selecting a volume for allocation. These processes are by definition asynchronous and hence can be simultaneously executed. As allocation requests are processed, some allocations may, due to the asynchronous nature of the multihost environment, be preempted. That is, the space available in a given storage volume may be partially allocated before a given allocation request is processed. When a volume is fragmented, such allocations can, due to fragmentation, cause a residual of available or allocatable data storage space with individual extents that do not satisfy the allocation request requirements. By treating such a fragmented volume as having logically contiguous allocatable space, and by absolutely measuring that space on each allocation request, failures are avoided.

The allocation process in each of the individual hosts are is and asynchronously initiated by each processor. The process involves screening data set parameters against eli...