Research Topics
ANSWER:
File-System Structure
- Hard disks have two important properties that make them suitable for secondary storage of files in file systems: (1) Blocks of data can be rewritten in place, and (2) they are direct access, allowing any block of data to be accessed with only ( relatively ) minor movements of the disk heads and rotational latency. ( See Chapter 12 )
- Disks are usually accessed in physical blocks, rather than a byte at a time. Block sizes may range from 512 bytes to 4K or larger.
- File systems organize storage on disk drives, and can be viewed as a layered design:
- At the lowest layer are the physical devices, consisting of the magnetic media, motors & controls, and the electronics connected to them and controlling them. Modern disk put more and more of the electronic controls directly on the disk drive itself, leaving relatively little work for the disk controller card to perform.
- I/O Control consists of device drivers, special software programs ( often written in assembly ) which communicate with the devices by reading and writing special codes directly to and from memory addresses corresponding to the controller card's registers. Each controller card ( device ) on a system has a different set of addresses ( registers, a.k.a. ports ) that it listens to, and a unique set of command codes and results codes that it understands.
- The basic file system level works directly with the device drivers in terms of retrieving and storing raw blocks of data, without any consideration for what is in each block. Depending on the system, blocks may be referred to with a single block number, ( e.g. block # 234234 ), or with head-sector-cylinder combinations.
- The file organization module knows about files and their logical blocks, and how they map to physical blocks on the disk. In addition to translating from logical to physical blocks, the file organization module also maintains the list of free blocks, and allocates free blocks to files as needed.
- The logical file system deals with all of the meta data associated with a file ( UID, GID, mode, dates, etc ), i.e. everything about the file except the data itself. This level manages the directory structure and the mapping of file names to file control blocks, FCBs, which contain all of the meta data as well as block number information for finding the data on the disk.
- The layered approach to file systems means that much of the code can be used uniformly for a wide variety of different file systems, and only certain layers need to be filesystem specific. Common file systems in use include the UNIX file system, UFS, the Berkeley Fast File System, FFS, Windows systems FAT, FAT32, NTFS, CD-ROM systems ISO 9660, and for Linux the extended file systems ext2 and ext3 ( among 40 others supported. )
ANSWER:
- Mac OS file names compatibility:
Mac OS9 supports HFS and HFS+ File system and may be up to 31 characters long.
Max OSX supports HFS+ and UFS (Unix File System) and may be up to 255 characters long.
Colon ":" is used as a directory separator.
File and folder names may use any characters except colon ":" and forward slash "/" which is used for directory separators on OS9 and OSX.
Filenames preserves case but are not case sensitive.
- Windows file names compatibility:
Windows uses two different file systems: File Allocation Table (FAT) file system and New Technology File System (NTFS).
File and folder names may be up to 255 characters long on FAT file systems and 256 characters long on NTFS. Full pathnames may be up to 260 characters long.
There is a limit to the full pathnames of 260 characters long.
Backslash "\" is used as the directory separator.
File and directory names may not contain the special characters: "/\*?<>|:
Periods are allowed in file and directory names, but not as the last character. for example "letter.doc" is legal but "letterdoc." is illegal file name.
Filenames preserves case but are not case sensitive.
- UNIX file names compatibility:
Unix base operating systems mostly use the UFS (Unix File System).
File and folder names may be up to 255 characters long.
Forward slash"/" is used as the directory separator.
File and directory names may not contain the special characters: space and /
Filenames preserves case and are case sensitive.
Exercises
- Explain in your own words why file deallocation is important and what would happen if it did not occur on a regular basis.
Answer:
Deallocation is important because it frees up resources for other jobs, processes, files, etc. If deallocation did not occur on a regular basis then there would be limited services a computer can perform at one time or over a period of time.
2. Describe how the File Manager allocates a file to a single user. List steps that you think would be followed and explain your reasoning.
Answer:
Responsibilities of the File Manager
1. Discuss how the File Manager’s responsibilities are quite complex and involve the physical components and information resources of the system, as well as its policies for storing and distributing files.
2. Outline the four tasks that the File Manager follows to carry out its responsibilities:
a. Keep track of where each file is stored.
b. Use a policy that will determine where and how the files will be stored, making sure to efficiently use available storage space and provide efficient access to files.
c. Allocate each file when a user has been cleared for access to it, and then record its use.
d. Deallocate the file when the file is to be returned to storage, and communicate its availability to others who may be waiting for it.
3. Point out that the File Manager’s policy determines where each file is stored and how the system and users will be able to access them through commands that are independent from device details.
4. Discuss the two factors the File Manager’s policy utilizes to determine who may have access to what material: flexibility of access to the information and its subsequent protection. Explain that the File Manager allows access to shared files, provides distributed access, and allows users to browse through public directories. Additionally, the operating system must protect its files against system malfunctions and provide security checks through account numbers, passwords, and lockwords to preserve the integrity of the data and safeguard against tampering.
5. Describe the steps involved in the file allocation and deallocation processes.
3. Is device independence important to the file manager ? Why or why not ? Describe the consequences if that were not the case.
Answer:
No, A file manager or file browser is a computer program that provides a user interface to work with file systems. The most common operations performed on files or groups of files are: create, open, edit, view, print, play, rename, move, copy, delete, search/find, and modify file attributes, properties and file permissions. Files are typically displayed in a hierarchy. Some file managers contain features inspired by web browsers, including forward and back navigational buttons.
Some file managers provide network connectivity via protocols, such as FTP, NFS, SMB or WebDAV. This is achieved by allowing the user to browse for a file server (connecting and accessing the server's file system like a local file system) or by providing its own full client implementations for file server protocols.
4. Do you think file retrieval is different on a menu-driven system and a command-driven system? Explain your answer and describe any differences between two . Give an example of when each would be preferred over the other.
Answer:
Refers to programs and operating systems that accept commands in the form of special words or letters. In contrast, programs that allow you to choose from a list of options in a menu are said to be menu driven. Command-driven software is often more flexible than menu-driven software, but it is more difficult to learn.
5. Imagine one real life example of each: a multi-file volume and a multi-volume file. Include a description of the media used for storage and a general description of the data in the file.
Answer:
Most large databases are multi-volume files. Most disk drives are multi-file volumes.
6. As described in this chapter, files can be formatted with fixed-lenght fields or variable-length fields. In your opinion, would it be feasible to combine both formats in a single disk? Explain the reasons for your answer.
Answer:
No, it is not feasible to actually combine both formats in a single disk, because both are used in different areas .
7. Explain why it's difficult to support direct access to files with variable-length records. Suggest a method for handling this type of file if direct access is recquired.
Answer:
It is hard to calculate exactly where the record is located as what is given . If such is required then one should use the fixed-length fields because it can be easily accessed directly.
8. Give an example of the names of three files from your own PC that do not reside at the root or master directory. For each file, list both the relative filename and it's complete filename.
Answer:
- Directories (/usr "user" or /dev "device")
- Sub-directories (/usr/local)
- Files: data, devices, links, etc. (/usr/local/readme.txt or /dev/hda1, which is the hard disk device)
