File System Terminology

Block The basic unit of disk storage. It is 512 bytes in size, the same size as a VAX memory page or the Alpha pagelet. Making the disk and memory units the same size simplifies the virtual memory management code.

Volume The basic disk storage medium under Files-11. It is treated as an array of logical blocks, which must be randomly addressable. Today, this usually means a single disk drive, or sometimes several separate drives bound by hardware in such a way as to act as a single unit. Historically, the concepts of volume and disk drive were different in that a large stationary disk drive could contain a removable disk volume.

This is less often the case today. Currently, the concept of a floppy diskette drive provides a close approximation: The device (the diskette drive) remains fixed and retains the same device name regardless of what volume (the floppy diskette) currently resides in it.

Logical Block A disk block numbered relative to the start of the disk. The file system treats a disk volume as a contiguous array of logical blocks, numbered zero through n –1, where nis the number of blocks on the disk. This number is not concerned with any bad-block replacement, number of actual drives, surfaces, etc. This allows most software above the disk driver to treat a disk as an abstract device with no media defects.

Physical Block A block at a certain surface/track/sector on a drive. When presented with a logical block number, the disk driver calculates the physical disk block that contains the data.

Virtual Block A block assigned to a file, relative to the beginning of that file, starting with virtual block 1. A virtual block always has an associated logical block, but a logical block has an associated virtual block only if it is assigned to a file. Keep in mind that whereas logical blocks start with 0, virtual blocks start with 1.

Bad-Block Replacement Most disk surfaces contain media defects. Disks come from the factory with internal lists of blocks that cannot reliably hold information. Each drive has some empty spare blocks to be used as replacements for bad blocks discovered later. Some time after the disk enters service, additional blocks may fail. These are added to the list of bad blocks and are assigned a replacement block. This process is sometimes handled by operating system software, but increasingly, with modern disks it is handled within the disk unit.

Cluster Factor The unit of space allocation on a volume, expressed in blocks. For example, a certain disk may have a cluster factor of three. When blocks are allocated, they are allocated in units of three blocks at a time. The system manager selects this number as a balance between a reduced number of allocation operations and unused (wasted) blocks at the ends of files. Also known as cluster size.

Disk Driver A piece of operating system software that controls the actual operation of a disk drive. The disk driver is not aware of a file system structure on disk; it moves raw blocks of data as directed by the file system.

File Header An entry in INDEXF.SYS describing a file. Most files have one header, but files with many extents (fragments) and/or files with many ACL entries may have more than one header due to space limitations in the first header.

Seek The mechanical movement of disk heads to a different track. Tracks are the concentric rings of data storage areas on a disk surface.

File Fragmentation The process of a file being stored to discontiguous ranges of logical blocks (parts of the file being scattered over the disk). This is an expected by-product of file system operation over time, but it extracts a performance penalty. Retrieving all parts of the file requires extra mechanical movements of the disk hardware, slowing performance.

Retrieval Pointers Pairs of numbers describing the location and size of each file extent (part of the file). They are stored in the file header.



Getting Started with OpenVMS(c) A Guide for New Users
Getting Started with OpenVMS: A Guide for New Users (HP Technologies)
ISBN: 1555582796
EAN: 2147483647
Year: 2005
Pages: 215

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