Chapter 6. File Systems

   

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A file system provides some essential functionality, including the following:

• Maintaining data integrity while meeting user needs in terms of providing functionality to create, delete, read, and write files.

• Providing high throughput and performance while showing resiliency to system and disk failures.

• Providing support for a wide variety of devices, such as fixed disks and removable disks, and each of these with a wide range of size and performance.

• Providing an abstraction for applications such that they need not bother about the physical characteristics of the underlying storage device or where exactly within that device the data of interest to the application for a given I/O resides. Applications simply treat a file as a series of linear bytes and seek within that series of bytes, doing read or write operations at will.

• Providing an abstraction that allows an application to organize files into a hierarchy ”for example, a directory containing other files and directories. In practice, the hierarchy is really an illusion maintained by the file system, since all data is just on the disk in random clusters.

• Providing a security layer for data; for example, NTFS enforces the mandate that only authorized users are allowed access to appropriate resources such as files, directories, or volumes .

• Providing support for concurrency ”that is, allowing multiple users and multiple I/Os from each user ”while also providing a serialization and locking mechanism that allows a particular user I/O operation to uniquely own a file or portion of a file. The file system must also recover from crashes of applications that may own file system resources such as a lock on a byte range of a file.

File systems do not reside in a vacuum ; they reside on media of some sort . Although optical media such as CDs and DVDs have file systems as well, this book is about enterprise storage on Windows, and enterprise storage deals primarily with disks. Hence it is essential that one understand how Windows NT organizes disks. The first part of this chapter is dedicated to explaining how Windows NT organizes disks in terms of basic disks, dynamic disks, and their relationships to partitions and volumes.

After this discussion, the chapter moves on to describe significant details of the NTFS file system. This segment includes a description of important internal data structures, such as the master file table, as well as a description of features such as compression, the change log journal, link tracking, reparse points, and the encrypting file system.

The last part of the chapter is dedicated to describing SAN file systems, including advantages of a SAN file system and the technical issues in building one. Some vendor offerings in this area are also briefly examined.

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