-操作系统精髓与设计原理(第五版)+课后题答案1
CHAPTER 12 FILE MANAGEMENT
Review Questions
12.1 A field is the basic element of data containing a single value. A record is
a collection of related fields that can be treated as a unit by some application program.
12.5 Pile: Data are collected in the order in which they arrive. Each record
consists of one burst of data. Sequential file: A fixed format is used for records. All records are of the same length, consisting of the same number of fixed-length fields in a particular order. Because the length and position of each field is known, only the values of fields need to be stored; the field name and length for each field are attributes of the file structure. Indexed sequential file: The indexed sequential file maintains the key characteristic of the sequential file: records are organized in sequence based on a key field. Two features are added; an index to the file to support random access, and an overflow file. The index provides a lookup capability to reach quickly the vicinity of a desired record. The overflow file is similar to the log file used with a sequential file, but is integrated so that records in the overflow file are located by following a pointer from their predecessor record. Indexed file: Records are
accessed only through their indexes. The result is that there is now no restriction on the placement of records as long as a pointer in at least one index refers to that record. Furthermore, variable-length records can be employed. Direct, or hashed, file: The direct file makes use of hashing on the key value.
Problems
B
12.1 Fixed blocking: F = largest integer
R
When records of variable length are packed into blocks, data for marking
the record boundaries within the block has to be added to separate the records. When spanned records bridge block boundaries, some reference to the successor block is also needed. One possibility is a length indicator preceding each record. Another possibility is a special separator marker between records. In any case, we can assume that each record requires a marker, and we assume that the size of a marker is about equal to the size of a block pointer [WEID87]. For spanned blocking, a block pointer