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Top 10 List of Week 03

1. What is file?
   A file is an abstract data type defined and implemented by the operating system. It is a sequence of logical records. A logical record may be a byte, a line (of fixed or variable length), or a more complex data item. The operating system may specifically support various record types or may leave that support to the application program.

2. File System, Single-Level and Two-Level Directory
   Within a file system, it is useful to create directories to allow files to be organized. A single-level directory in a multiuser system causes naming problems, since each file must have a unique name. A two-level directory solves this problem by creating a separate directory for each user’s files. The directory lists the files by name and includes the file’s location on the disk, length, type, owner, time of creation, time of last use, and so on.

3. Generalization of Two Level Directory
   The natural generalization of a two-level directory is a tree-structured directory. A tree-structured directory allows a user to create subdirectories to organize files. Acyclic-graph directory structures enable users to share subdirectories and files but complicate searching and deletion. A general graph structure allows complete flexibility in the sharing of files and directories but sometimes requires garbage collection to recover unused disk space.

4. Remote File System
   Remote file systems present challenges in reliability, performance, and security. Distributed information systems maintain user, host, and access information so that clients and servers can share state information to manage use and access.

5. Apple File System
   Apple File System (APFS) is a good example of such a design. Its goal is to run on all current Apple devices, from the Apple Watch through the iPhone to the Mac computers. Creating a file system that works in watchOS, I/Os, tvOS, and macOS is certainly a challenge. APFS is feature-rich, including 64-bit pointers, clones for files and directories, snapshots, space sharing, fast directory sizing, atomic safe-save primitives, copy-on-write design, encryption (single- and multi-key), and I/O coalescing. It understands NVM as well as HDD storage

6. Directory Management
   Directory-management routines must consider efficiency, performance, and reliability. A hash table is a commonly used method, as it is fast and efficient. Unfortunately, damage to the table or a system crash can result in inconsistency between the directory information and the disk’s contents.

7. Free Space Allocation Method
   Free-space allocation methods also influence the efficiency of disk-space use, the performance of the file system, and the reliability of secondary storage. The methods used include bit vectors and linked lists. Optimizations include grouping, counting, and the FAT, which places the linked list in one contiguous area.

8. What should be considered in directory-management routines?
   Directory-management routines must consider efficiency, performance, and reliability. A hash table is a commonly used method, as it is fast and efficient. Unfortunately, damage to the table or a system crash can result in inconsistency between the directory information and the disk’s contents.

9. Advantages of Network File System
   • Data accessed by all users can be kept on a central host, with clients mounting this directory at boot time. For example, you can keep all user accounts on one host, and have all hosts on your network mount /home from that host. If installed alongside with NIS, users can then log into any system, and still work on one set of files.
   • Data consuming large amounts of disk space may be kept on a single host. For example, all files and programs relating to LaTeX and METAFONT could be kept and maintained in one place.
   • Administrative data may be kept on a single host. No need to use rcp anymore to install the same stupid file on 20 different machines.

10. Distributed Information System: DNS
    Client–server facilities do not natively share information, but a distributed information system such as DNS can be used to allow such sharing, providing a unified user name space, password management, and system identification. For example, Microsoft CIFS uses active directory, which employs a version of the Kerberos network authentication protocol to provide a full set of naming and authentication services among the computers in a network.