Wednesday, August 17, 2011

Some details about Common disk formats

Partitioned disk is available only after formatting. The common disk format includes FAT16, FAT32, NTFS, ext2 and ext3, etc.

In 1984, IBM released the PC AT, which featured a 20 MB hard disk. Microsoft introduced MS-DOS 3.0 in parallel. Cluster addresses were increased to 16-bit, allowing for up to 65,517 clusters per volume, and consequently much greater file system sizes, at least in theory. However, the maximum possible number of sectors and the maximum (partition, rather than disk) size of 32 MB did not change. Therefore, although technically already "FAT16", this format was not what today is commonly understood as FAT16. With the initial implementation of FAT16 not actually providing for larger partition sizes than FAT12, the early benefit of FAT16 was to enable the use of smaller clusters, making disk usage more efficient, particularly for files several hundred bytes in size, which were far more common at the time. Also, the introduction of FAT16 actually did bring an increase in the maximum partition size under MS-DOS, since the implementation of FAT12 for hard disks in MS-DOS 2.0 was limited to 15 MB.

In order to overcome size limit of FAT16, while at the same time allowing DOS real mode code to handle the format, and without reducing available conventional memory unnecessarily, Microsoft implemented a next generation, known as FAT32. Cluster values are represented by 32-bit numbers, of which 28 bits are used to hold the cluster number, for a maximum of approximately 268 million clusters. This allows for drive sizes of up to 8 TiB with 32 KiB clusters, but the boot sector uses a 32-bit field for the sector count, limiting volume size to 2 TB on a hard disk with 512 byte sectors.

NTFS (New Technology File System) is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista and Windows 7. NTFS supersedes the FAT file system as the preferred file system for Microsoft's Windows operating systems. NTFS has several improvements over FAT and HPFS (High Performance File System) such as improved support for metadata and the use of advanced data structures to improve performance, reliability, and disk space utilization, plus additional extensions such as security access control lists (ACL) and file system journaling.

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