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HP OpenVMS Systemsask the wizard |
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The Question is: Where may I find a tool that can make some kind of disk partition on SCSI disks? The Answer is : Strictly speaking, you cannot create "partitions" on OpenVMS disk volumes (using the term "partition" in the DOS or Windows FAT sense) without the assistance of the storage hardware. (A DOS partition is a set of data structures that permit multiple file system instances to coexist on the same disk.) Storage controllers (eg: the HSZ70, HSZ80, HSG80, HSJ50) allow you to divide physical disk volumes into partitions, which then appear as independent disk volumes to the host OpenVMS system. This is, potentially, an expensive solution to your problem. There are tools available that can create "logical" or "virtual" disk volumes on a physical disk volume, akin to the IBM DASD model or the DOS partition model. On OpenVMS, these "logical" volumes are implemented as a device driver, and particularly as a device driver which presents a disk-like device interface to the operating system, but that manages a container file on the physical disk, or that manages one or more ranges of disk logical blocks. Two such tools are LDDRIVER and VDDRIVER (and variants), both of which are available on the OpenVMS Freeware CD-ROM distribution. These tools can operate on any disk device (other than a system disk), and not just on a SCSI disk. Among available commercial products that provide this is the RAID Software for OpenVMS package -- you can create a RAID 0 or 0+1 set out of a single disk drive, and can then partition the virtual RAID device as required. Unlike the DOS partition approach, the low-level OpenVMS bootstrap device drivers are not capable of bootstrapping off a software-partitioned disk. Data disks can be partitioned. Though a bootstrap is not possible from it, disk container files can certainly exist on the system disk as well as on data disks. The OpenVMS bootstrap drivers are potentially (often) capable of bootstrapping off a controller-based partition. There are advantages and disadvantage to container files and to software partitioning in general. The biggest disadvantage is that they are somewhat slower than accessing the disk directly; there is more driver software overhead involved. The biggest advantage is that the container file is, really, just another file, and it can be moved around, backed up, etc., without having to copy the data in the container file. Container files, logical-block-based drivers, and controller-based disk partitions -- because these all present the appearance of a smaller total disk capacity -- can also be used to permit smaller disk cluster factors. (The disk cluster factor is the minimal unit of storage allocation on a disk.) This can potentially increase the available amount of useful storage, particularly when many smaller files exist on a large disk. (The average "wasted" space on a disk can be calculated based on half of the disk cluster factor times the number of files on the disk, on the assumption that half of the last disk cluster allocated to each file is unused.) OpenVMS V7.2 and later permit smaller disk cluster factors and these smaller cluster factors can make better use of storage when larger numbers of files are resident on larger disks. Prior to V7.2, the minimum disk cluster factor scales with the size of the volume due to the size of the storage bitmap, and cluster factors of 18 and 36 blocks are not unusual. With V7.2 and later, a disk cluster factor of 1 is permitted on a volume of up to 137 GB in capacity, at which capacity the permitted minimum disk cluster factor doubles to 2.
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