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In the following section, we ll look at the requirements that drove the design of NTFS. Then in the subsequent section, we ll examine the advanced features of NTFS.

From the start, NTFS was designed to include features required of an enterprise-class file system. To minimize data loss in the face of an unexpected system outage or crash, a file system must ensure that the integrity of its metadata is guaranteed at all times; and to protect sensitive data from unauthorized access, a file system must have an integrated security model. Finally, a file system must allow for software-based data redundancy as a low-cost alternative to hardware-redundant solutions for protecting user data. In this section, you ll find out how NTFS implements each of these capabilities.

To address the requirement for reliable data storage and data access, NTFS provides file system recovery based on the concept of an atomic transaction. Atomic transactions are a technique for handling modifications to a database so that system failures don t affect the correctness or integrity of the database. The basic tenet of atomic transactions is that some database operations, called transactions, are all-or-nothing propositions. (A transaction is defined as an I/O operation that alters file system data or changes the volume s directory structure.) The separate disk updates that make up the transaction must be executed atomically that is, once the transaction begins to execute, all its disk updates must be completed. If a system failure interrupts the transaction, the part that has been completed must be undone, or rolled back. The rollback operation returns the database to a previously known and consistent state, as if the transaction had never occurred. NTFS uses atomic transactions to implement its file system recovery feature. If a program initiates an I/O operation that alters the structure of an NTFS volume that is, changes the directory structure, extends a file, allocates space for a new file, and so on NTFS treats that operation as an atomic transaction. It guarantees that the transaction is either completed or, if the system fails while executing the transaction, rolled back. The details of how NTFS does this are explained in the section NTFS Recovery Support later in the chapter. In addition, NTFS uses redundant storage for vital file system information so that if a sector on the disk goes bad, NTFS can still access the volume s critical file system data.

Security in NTFS is derived directly from the Windows object model. Files and directories are protected from being accessed by unauthorized users. (For more information on Windows security, see 6.) An open file is implemented as a file object with a security descriptor stored on disk as a part of the file. Before a process can open a handle to any object, including a file object, the Windows security system verifies that the process has appropriate authorization to

do so. The security descriptor, combined with the requirement that a user log on to the system and provide an identifying password, ensures that no process can access a file unless given specific permission to do so by a system administrator or by the file s owner. (For more information about security descriptors, see the section Security Descriptors and Access Control in 6, and for more details about file objects, see the section Opening Devices in 7.) Data Redundancy and Fault Tolerance In addition to recoverability of file system data, some customers require that their own data not be endangered by a power outage or catastrophic disk failure. The NTFS recovery capabilities do ensure that the file system on a volume remains accessible, but they make no guarantees for complete recovery of user files. Protection for applications that can t risk losing file data is provided through data redundancy. Data redundancy for user files is implemented via the Windows layered driver model (explained in 7), which provides fault-tolerant disk support. NTFS communicates with a volume manager, which in turn communicates with a hard disk driver to write data to disk. A volume manager can mirror, or duplicate, data from one disk onto another disk so that a redundant copy can always be retrieved. This support is commonly called RAID level 1. Volume managers also allow data to be written in stripes across three or more disks, using the equivalent of one disk to maintain parity information. If the data on one disk is lost or becomes inaccessible, the driver can reconstruct the disk s contents by means of exclusive-OR operations. This support is called RAID level 5. (See 8 for more information on striped volumes, mirrored volumes, and RAID-5 volumes.)