is the software responsible for creating, deleting, modifying, and controlling access to files—as well as for managing the resources used by the files
The File Manager
Responsibilities of the File Manager
- Keep track of where each file is stored.
- Use a policy that will determine where and how the files will be stored, making sure to efficiently use the available storage space and provide efficient access to the files.
- Allocate each file when a user has been cleared for access to it, then record its use.
- Deallocate the file when the file is to be returned to storage, and communicate its availability to others who may be waiting for it.
How does a computer system allocate a file
The computer system allocates a file by activating the appropriate secondary storage device and loading the file into memory while updating its records of who is using what file
is a group of related bytes that can be identified by the user with a name, type, and size.
field
How does a computer system deallocate a file
Finally, the File Manager deallocates a file by updating the file tables and rewriting the file (if revised) to the secondary storage device. Any processes waiting to access the file are then notified of its availability.
is a group of related fields.
record
is a group of related records that contains information to be used by specific application programs to generate reports.
file
This type of file contains data and is some times called a _________ because it has no connections to other files; unlike databases, it has no dimensionality.
flat file
appears to the File Manager to be a type of file, but ________ are more complex because they’re actually groups of related files that are interconnected at various levels to give users flexibility of access to the data stored
database
contain instructions and data files contain data; but as far as storage is concerned, the File Manager treats them exactly the same way.
Program files
are special files with listings of filenames and their attributes. Data collected to monitor system performance and provide for system accounting is collected into files. In fact, every program and data file accessed by the computer system, as well as every piece of computer software, is treated as a file
Directories
The user communicates with the File Manager, which responds to what specific commands
OPEN
DELETE
RENAME
COPY
CREATE
OPEN NEW
when a user’s program issues a command to read a record from a disk the READ instruction has to be decomposed into the following steps:
- Move the read/write heads to the cylinder or track where the record is to be found.
- Wait for the rotational delay until the sector containing the desired record passes under the read/write head.
- Activate the appropriate read/write head and read the record.
- Transfer the record to main memory.
- Set a flag to indicate that the device is free to satisfy another request.
Each storage unit, whether it’s removable or not, is considered a
volume
each volume can contain several files
multifile volumes
files that are extremely large and are contained in several volumes
multivolume files
is stored immediately after the volume descriptor and lists the names and characteristics of every file contained in that volume
master file directory (MFD)
master file directory major disadvantages are:
- It would take a long time to search for an individual file, especially if the MFD was organized in an arbitrary order.
- If the user had more than 256 small files stored in the volume, the directory space (with a 256 filename limit) would fill up before the disk storage space filled up. The user would then receive a message of “disk full” when only the directory itself was full.
- Users couldn’t create subdirectories to group the files that were related.
- Multiple users couldn’t safeguard their files from other users because the entire directory was freely made available to every user in the group on request.
- Each program in the entire directory needed a unique name, even those directories serving many users, so only one person using that directory could have a program named Program1
is created when a user opens an account to access the computer system
subdirectory
Information typically included in a file descriptor includes the following:
- Filename—within a single directory, filenames must be unique; in some operating systems, the filenames are case sensitive
- File type—the organization and usage that are dependent on the system (for example, files and directories)
- File size—although it could be computed from other information, the size is kept
here for convenience
* File location—identification of the first physical block (or all blocks) where the file is stored
- Date and time of creation
- Owner
- Protection information—access restrictions, based on who is allowed to access the file and what type of access is allowed
- Record size—its fixed size or its maximum size, depending on the type of record
The two components common to many filenames
a relative filename and an extension
are the most common because they’re the easiest to access directly. That’s why they’re ideal for data files.
Fixed-length records
they don’t leave empty storage space and don’t truncate any characters, thus eliminating the two disadvantages of fixed-length records
Variable-length records
On magnetic disks (hard drives), files can be organized in one of several ways:
sequential record organization
direct record organization
indexed sequential record organization
-
Chapter 1103
-
Chapter 25
-
Review Questions13
-
Chapter 361
-
Chapter 441
-
Key Terms79
-
Chapter 527
-
Chapter 5 key terms19
-
Chapter 3 Key Terms32
-
Chapter 4 Key Terms37
-
Chapter 2 Key Terms18
-
Chapter 1 Key Terms32
-
Chapter 645
-
Chapter 6 Key Terms25
-
Chapter 756
-
Chapter 7 Key Terms42
-
Chapter 8 Key Terms33
-
Chapter 847
