Day 29 File System

NTFS vs. FAT FAT16 – 16bit addresses NTFS – uses 64-bit addresses 216 * 1KB/block => 216KB = 64GB Increase from 1KB to 5KB => more internal fragmentation NTFS – uses 64-bit addresses 264 * 1KB/cluster = (216)4KB It is not recommended to use NTFS on a volume that is smaller than approximately 400 MB, because of the amount of space overhead involved in NTFS. This space overhead is in the form of NTFS system files that typically use at least 4 MB of drive space on a 100 MB partition.

NTFS vs. FAT NTFS has Access Control Lists in order to enforce security on individual items. An open file is implemented as a file object that has a security descriptor that defines its security attributes

NTFS vs. FAT FAT uses 8-bit ASCII code for filenames. NTFS uses 16-bit UNICODE for filenames

NTFS vs. FAT In FAT, there are no mechanisms for fault tolerance or data recoverability. NTFS has built-in transactional logging to keep track of changes being made to the file system and for recovery from a crash.

NTFS vs. FAT It is not recommended to use NTFS on a volume that is smaller than approximately 400 MB, because of the amount of space overhead involved in NTFS. This space overhead is in the form of NTFS system files that typically use at least 4 MB of drive space.

FAT12 file system – Disk layout Figure obtained from the CS324 course web-site from Brigham Young University

Boot sector in FAT12 (BPB – BIOS Parameter Block)

Directory entry

How to use the file allocation table How to use the file allocation table? – Linked list allocation with an index Figure obtained from the CS324 course web-site from Brigham Young University

FAT packing A FAT table has an entry for each sector and contains pointers to these sector numbers. A sector address in FAT12 is a 12-bit value (up to 4096 sectors, only use 2848 in FAT12). Typically, we store 8-bit, 16-bit, 32-bit values in memory. Using 16-bits for each 12-bit value would result in a waste of 4 X 2848 = 12 kilobytes out of 1.44MB of space.

FAT packing Take two 12-bit values and fit them into three bytes i.e. 24 bits. No waste

References William Stallings, “Operating Systems: Internals and Design Principles”, 5th edition. A.S. Tanenbaum and A.S. WoodHull,”Operating Systems: Design and Implementation”,2nd edition A. Silberschatz, G. Gagne and P.B. Galvin,”Operating Systems Concepts”, 7th edition. http://www.windowsitpro.com/Windows/Articles/ArticleID/3455/pg/2/2.html http://students.cs.byu.edu/~cs345ta/labs/fall03_specs/lab_fat_help.htm#Directory%20structures%20and%20their%20fields