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Full-Bandwidth Fragmentation-Free Method of Disk Storage Allocation

IP.com Disclosure Number: IPCOM000047501D
Original Publication Date: 1983-Nov-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Bryant, RM: AUTHOR

Abstract

This method of allocating tracks on secondary storage disks enables sequential files to be transferred at full disk bandwidth without external storage fragmentation problems. The method is applicable to disk drives that have at least two separate actuator arms, such as the IBM 3380, for example. Alternate blocks of a file are allocated to alternate arms of the drive in a way that allows blocks read from one arm to be completely overlapped with seeks done on the other arm. In an exemplary application of this method, a block size of three tracks is utilized. Transfer time of 50 ms. for such a block exceeds the maximum 3380 seek time of 38 ms.

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Full-Bandwidth Fragmentation-Free Method of Disk Storage Allocation

This method of allocating tracks on secondary storage disks enables sequential files to be transferred at full disk bandwidth without external storage fragmentation problems. The method is applicable to disk drives that have at least two separate actuator arms, such as the IBM 3380, for example. Alternate blocks of a file are allocated to alternate arms of the drive in a way that allows blocks read from one arm to be completely overlapped with seeks done on the other arm. In an exemplary application of this method, a block size of three tracks is utilized. Transfer time of 50 ms. for such a block exceeds the maximum 3380 seek time of 38 ms. Alternatively, one could use a block size of one track provided that subsequent blocks of a file to be fetched from the same arm of a device are not separated by more than about 250 cylinders (14 ms. seek time). Such allocation could be achieved by splitting the free list into four parts, one part for each 222 contiguous cylinders serviced by the arm (there being 885 cylinders per arm on a 3380 disk). It is assumed that full track reads from alternate arms can occur with essentially no intervening delay. This requires the tracks accessed by the two arms to be offset in exactly the same way the arms are. For the 3380 disk, this would mean that the starting track locations would have to be 180 degrees apart. Separate disks could not be used in the manner describ...