Understanding file systems

What is file system?

Any computer file is stored on some kind of storage with a given capacity. Actually, each storage is a linear space to read or both read and write 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 – single byte). Any file saved to the storage takes a number of these cells.

Generally, computer storages use a pair of sector and in-sector offset to reference any byte of information on the storage. The sector is a group of bytes (usually 512 bytes) that is a minimum addressable unit of the physical storage. For example, byte 1030 on a hard disk will be referenced as sector #3 and offset in sector 16 bytes ([sector]+[sector]+[16 bytes]). This scheme is applied to optimize storage addressing and use a smaller number to reference any portion of information on the storage.

To omit the second part of the address (in-sector offset), files are usually stored starting from the sector start and occupy all whole sectors (e.g.: 10-byte file occupies the whole sector, 512-byte file also occupies the whole sector, at the same time, 514 byte file occupies two whole sectors).

Each file is stored to 'unused' sectors and can be read then by known position and size. However, how do we know what sectors are used or unused? Where are file size and position stored? Where is file name? These answers give us the file system.

As a whole, file system is a structured data representation and a set of metadata that describe the stored data. File system can not only serve for the purposes of the whole storage but also be a part of an isolated storage segment – disk partition. Usually the file system operates blocks, not sectors. File system blocks are groups of sectors that optimize storage addressing. Modern file systems generally use block sizes from 1 up to 128 sectors (512-65536 bytes). Files are usually stored from the start of a block and take entire blocks.

Immense write/delete operations to file system cause file system fragmentation. As a result files aren't stored as whole fragments anymore and are divided into fragments. For example, a storage is entirely taken by files with size about 4 blocks (e.g. pictures collection). User wants to store a file that will take 8 blocks and therefore deletes the first and the last file. By doing this he releases 8 blocks, however, the first segment is near to the storage start, and the second – to the storage end. In this case 8-block file will be split into two parts (4 blocks for each part) and will take free space 'holes'. The information about both fragments, which are parts of a single file, will be stored to file system.

In addition to user files the file system also stores its own parameters (such as block size), file descriptors (that include file size, file location, its fragments etc.), file names and directory hierarchy. It may also store security information, extended attributes and other parameters.

To comply with diverse requirements as to storage performance, stability and reliability there exists a great variety of file systems each developed to serve certain user purposes.

Windows file systems

Microsoft Windows OS use 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):

FAT file system is one of the most simple types of file systems. It consists of file system descriptor sector (boot sector or superblock), file system block allocation table (referenced 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 defines file or file extended attributes (e.g. long file name). File record references the first block of file. Any next block can be found through block allocation table by using it as linked-list.

Block allocation table contains an array of block descriptors. Zero value indicates that the block is not used and non-zero – reference to the next block of the file or special value for file end.

The number in FAT12, FAT16, FAT32 stands for the number if bits used to enumerate file system block. This means that FAT12 may use up to 4096 different block references, FAT16 - 65536 and FAT32 - 4294967296. Actual maximum count of blocks is even less and depends on implementation of file system driver.

FAT12 was used for old floppy disks. FAT16 (or simply FAT) and FAT32 are widely used for flash memory cards and USB flash sticks. It is supported by mobile phones, digital cameras and other portable devices.

FAT or FAT32 is a file system, used on Windows-compatible external storages or disk partitions with size below 2GB (for FAT) or 32GB (for FAT32). Windows can not create FAT32 file system over 32GB (however Linux supports FAT32 up to 2TB).

NTFS (New Technology File System):

NTFS was introduced in Windows NT and at present is major file system for Windows. This is a default file system for disk partitions and the only file system that is supported for 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 file descriptor in Master File Table and file content. 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 quite large disk storages.

ReFS (Resilient File System):

ReFS is the latest development of Microsoft presently available for Windows 8 Servers. File system architecture absolutely differs from other Windows file systems and is mainly organized in form of B+-tree. ReFS has high tolerance to failures achieved due to new features included into the system. And, namely, Copy-on-Write (CoW): no metadata is modified without being copied; no data is written over the existing ones and rather into a new disk space. With any file modifications a new copy of metadata is created into any free storage space, and then the system creates a link from older metadata to the newer ones. As a result a system stores significant quantity of older backups in different places which provides for easy file recovery unless this storage space is overwritten.

MacOS file systems

Apple Mac OS operating system applies HFS+ file system, an extension to their own HFS file system that was used on old Macintosh computers.

HFS+ file system is applied to Apple desktop products, including Mac computers, iPhone, iPod, as well as Apple X Server products. Advanced server products also use Apple Xsan file system, clustered file system derived from StorNext or CentraVision file systems.

This file system except files and folders also stores Finder information about directories view, window positions etc.

Linux file systems

Open-source Linux OS always aimed to implement, test and use different concepts of file systems. Among huge amount of various file system types the most popular Linux file systems nowadays are:

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

ReiserFS - alternative Linux file system designed to store huge amount 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 this purpose.

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

JFS - file system developed by IBM for their powerful computing systems. JFS one usually stands for JFS, JFS2 is the second edition. Currently this file system is open-source and is implemented in most modern Linux distributions.

The concept of 'hard links' used in this kind of OS makes most Linux file systems similar in that the file name is not regarded as file attribute and rather defined as an alias for a file in certain directory. File object can be linked from many locations, even many times from the same directory under different names. This is one of the causes why recovery of file names after file deletion or file system damage can be difficult or even impossible.

BSD, Solaris, Unix file systems

The most common file system for these OS is UFS (Unix File System) also often referred to FFS (Fast File System – fast compared to a previous file system used for Unix). UFS is a source of ideas for many other file system implementations.

Currently UFS (in different editions) is supported by all Unix-family OS and is major file system of BSD OS and Sun Solaris OS. Modern computer technologies tend to implement replacements for UFS in different OS (ZFS for Solaris, JFS and derived file systems for Unix etc.).

Clustered file systems

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

Among such distributed file systems are:

ZFS - Sun company 'Zettabyte File System' - the new file system developed for distrubuted 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 - Rad Hat Linux 'Global File System'.

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

Common property of these file systems is distributed storages support, extensibility and modularity.