Questions and Answers
What is the limitation of the simplest view of sector organization?
It is not efficient for reading a series of sectors
What is the purpose of interleaving in sector organization?
To leave an interval of several physical sectors between logically adjacent sectors
What is a cluster in the context of sector organization?
A fixed number of contiguous sectors
What is the purpose of the File Allocation Table (FAT)?
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What is an extent in the context of sector organization?
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What happens when the disk controller is processing the received information?
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Why do we use clustering in sector organization?
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What happens when the number of extents in a file increases?
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What is the primary advantage of direct access storage devices (DASDs)?
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What is the purpose of a sector on a disk?
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What is the process of moving the arm that holds the read/write heads called?
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What determines the number of tracks on a surface of a disk?
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What is the advantage of accessing data on a single cylinder?
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What is the role of the buffer in the process of reading data from a disk?
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Why are floppy disks not suitable for storing large amounts of data?
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What is the relationship between the number of platters and the number of tracks per cylinder?
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Study Notes
Direct Access Storage Devices (DASDs)
- Disk drives are classified as DASDs because they allow direct access to data.
- This is in contrast to serial devices, which use media like magnetic tape and require sequential access.
Floppy Disks
- Floppy disks are inexpensive, but slow and have limited data storage capacity.
- They are suitable for backing up individual files or small amounts of data, and for transporting data.
Disk Storage
- Data is stored on the surface of one or more platters.
- The data is organized in successive tracks on the surface of the disk.
- Each track is divided into a number of sectors.
- A sector is the smallest addressable portion of a disk.
Sector Access
- When a read statement is called, the computer operating system:
- Finds the correct surface, track, and sector.
- Reads the entire sector into a buffer in memory.
- Finds the requested byte within that buffer.
Cylinders
- Tracks directly above and below one another form a cylinder.
- The significance of a cylinder is that all data on it can be accessed without moving the arm that holds the read/write heads.
- Moving the arm is called seeking, which is usually the slowest part of reading data from a disk.
Physical Placement of Sectors
- The simplest view is that sectors are adjacent, fixed-sized segments of a track that hold a file.
- However, physically, sectors may not be adjacent to improve performance.
- Interleaving is used to leave an interval of several physical sectors between logically adjacent sectors.
- Clusters are used to group contiguous sectors together to improve performance.
Clusters
- A cluster is a fixed number of contiguous sectors.
- The file manager uses a file allocation table (FAT) to map logical sectors to physical clusters.
- The FAT contains a list of all clusters in a file, ordered according to the logical order of the sectors they contain.
Extents
- An extent is a contiguous group of clusters on a disk.
- If a file consists of only one extent, it means that all its sectors are physically contiguous.
- As the number of extents in a file increases, the file becomes more fragmented, leading to slower performance.
