Understanding file systems

Presently, the computer market offers a variety of opportunities of storing huge amount of personal or corporate information in digital form. Storage devices include internal and external hard drives, USB flash drives, memory cards of photo/video cameras, complex RAID-systems etc. Actual documents, presentations, pictures, music, video, databases, email messages are stored in a form of files which may be place-consuming.

Any computer file is stored on a storage with given capacity. Actually, each storage is a linear space for reading or both reading and writing digital information. Each byte of information on the storage has its own offset from the storage start (address) and is referenced by this address. A storage can be presented as a grid with a set of numbered cells (each cell is a single byte). Any file that is saved to the storage gets these cells.

Microsoft Windows OS uses two major file systems: FAT, inherited from old DOS with its later extension FAT32, and widely-used NTFS file systems. Recently released ReFS file system was developed by Microsoft as a new generation file system for Windows 8 Servers.

FAT ( File Allocation Table ) is one of the simplest types of file systems. It consists of a file system descriptor sector (boot sector or superblock), a file system block allocation table (referred as File Allocation Table) and plain storage space to store files and folders. Files on FAT are stored in directories. Each directory is an array of 32-byte records, each defining file or file extended attributes (e.g. a long file name). File record attributes the first block of a file. Any next block can be found through a block allocation table by using it as a linked list.

NTFS ( New Technology File System ) was introduced in Windows NT and currently it is a major file system for Windows. This is the default file system for disk partitions and the only file system that supports disk partitions over 32GB. The file system is quite extensible and supports many file properties, including access control, encryption etc. Each file on NTFS is stored as a file descriptor in aMaster File Table and file content. A Master file table contains all information about the file: size, allocation, name etc. The first and the last sectors of the file system contain file system settings (boot record or superblock). This file system uses 48 and 64 bit values to reference files, thus, supporting disk storages with high capacity.

ReFS ( Resilient File System ) is the latest development of Microsoft currently available for Windows 8 Servers. The file system architecture absolutely differs from other Windows file systems and is mainly organized in a form of a B+-tree. ReFS has high tolerance to failures due to new features included into the system. And, namely, Copy-on-Write (CoW): no metadata is modified without being copied; data is not written over the existing data, but into new disk space. With any file modifications, a new copy of metadata is stored into free storage space, and then the system creates a link from older metadata to the newer one. Thus, the system stores significant quantity of older backups in different places providing easy file recovery unless this storage space is overwritten.

Apple's MacOS operating system applies HFS+ file system, an extension to their own HFS file system used on old Macintosh computers.

Open-source Linux OS aims at implementing, testing and using different concepts of file systems. The most popular Linux file systems include:

Ext2, Ext3, Ext4 - a “native” Linux file system. This file system falls under active developments and improvements. Ext3 file system is just an extension of Ext2 that uses transactional file writing operations with a journal. Ext4 is a further development of Ext3, extended with the support of optimized file allocation information (extents) and extended file attributes. This file system is frequently used as a "root" file system for most Linux installations.

ReiserFS - an alternative Linux file system for storing a huge number of small files.. It has good capability of files search and enables compact files allocation by storing file tails or small files along with metadata in order not to use large file system blocks for the same purpose.

XFS - a file system derived from SGI company and was initially used for company’s IRIX servers. Now XFS specifications are implemented in Linux. XFS file system has great performance and is widely used to store files.

JFS - a file system developed by IBM for the company’s powerful computing systems. JFS1 usually stands for JFS, JFS2 is the second release. Currently, this file system is open-source and implemented in most modern Linux versions.

The concept of “hard links” used in this kind of operating systems makes most Linux file systems similar in that the file name is not regarded as a file attribute and rather defined as an alias for a file in a certain directory. A file object can be linked from many locations, even multiply from the same directory under different names. This can lead to serious and even insurmountable difficulties in recovery of file names after file deletion or file system damage.

The most common file system for these operating systems is UFS (Unix File System) also often referred to as FFS (Fast File System).

Clustered file systems are used in computer cluster systems. These file systems support distributed storage.

ZFS - Sun company “Zettabyte File System” - a new file system developed for distributed storages of Sun Solaris OS.

Apple Xsan – the Apple company evolution of CentraVision and later StorNext file systems.

VMFS - “Virtual Machine File System” developed by VMware company for its VMware ESX Server.

GFS - Red Hat Linux “Global File System”.

JFS1 - the original (legacy) design of IBM JFS file system used in older AIX storage systems.

Common properties of these file systems include distributed storages support, extensibility and modularity.

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