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| Active Partition |
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Any partition marked as bootable in the partition table for an x86 (or 80x86 the generic name of a micro-processor architecture) computer. The operating system must have one primary partition of the primary disk as active to boot from. Under Windows the special system startup files that are required for Windows to load will reside on the C volume. This is called the boot partition. Windows does not need to be installed on the boot C partition, and if this is the case the partition that has the Windows directory will be the system partition. Having such an arrangement is beneficial for multiple installed operating systems. By default, Windows is installed on the C partition (Windows Disk Management (WDM) consol (or MMC snap-in) by design prohibits the system/boot drive letter being changed) - systemroot being pre-Windows NT C:\WINDOWS, Windows NT/2000 C:\WINNT and Windows XP and Windows Server 2003 C:\WINDOWS - and is both the boot and system partition. It is also possible to create a setup where by multiple operating systems each work on a separate hard disk drive and work independently of each other. There is no need to share the boot partition as each operating system will have their own system startup files, oblivious to each other. However, the independent multiple operating system setup requires the user to set the primary bootable partition for the target operating system active each time, if not already set as active. In one respect this setup will reduce multiple operating system interaction problems but it adds inconvenience. Using a boot manger can lessen the inconvenience caused.
On basic disks (using MBR-style or GPT-style partitioning), partitions are known as basic volumes, which include primary partitions and logical dos drives.
See: Active Volume, Basic Disk, Basic Volume, Boot Volume, MBR Disk, and Partition. |
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| Active Volume |
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An active volume is an area of storage on a hard disk drive that functions as though it were a physically separate disk – by a single drive specifier. A volume is created when a partition (before it can be used) is formatted with a filesystem, e.g., FAT or NTFS, with a drive letter assigned to it. For example, in My Computer and Windows Explorer, volumes appear as local disks. The active partition (any partition marked as bootable in the partition table for an x86 computer) must be a primary partition on a basic disk (or MBR basic disk). Under Windows the system startup files that are required for Windows to load will always be installed on the C volume, the active volume. This is called the boot volume. The systemroot does not need to be installed on the bootable C volume, and if this is the case the volume that has the systemroot will be the system volume. Having such an arrangement is beneficial for multiple installed operating systems. By default, Windows is installed on the C volume (Windows Disk Management (WDM) consol (or MMC snap-in) by design prohibits the system/boot drive letter being changed) - systemroot being pre-Windows NT C:\WINDOWS, Windows NT/2000 C:\WINNT and Windows XP and Windows Server 2003 C:\WINDOWS - and is both the boot and system volume.
On basic disks (using MBR-style or GPT-style partitioning), partitions are known as basic volumes, which include primary partitions and logical dos drives.
See: Active Partition, Basic Disk, Basic Volume, Boot Volume, MBR Disk, and Partition. |
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| Allocation Unit (or Cluster) |
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In data storage, in a filesystem, an allocation unit is the smallest amount of hard disk drive space that can be allocated to hold a file, e.g., 1,024, 2,048, 8,096 or higher for larger volumes. All filesystems used by Windows organise hard disk drives based on allocation units, which consist of one or more contiguous sectors. The smaller the allocation unit size, the more efficiently a disk stores information. If no allocation unit size is specified during formatting, Windows assigns the default size based on the size of the volume and the filesystem used. These defaults are selected to reduce the amount of space lost and the amount of fragmentation on the volume. Allocation unit is interchangeable with cluster.
See: Cluster (or Allocation Unit), and Sector. |
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| American Standard Code for Information Interchange (ASCII) |
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ASCII is a standard single-byte character-encoding (coding) scheme used for text-based data. ASCII uses designated 7-bit or 8-bit number combinations to represent either 128 or 256 possible characters. Standard ASCII uses 7 bits to represent all uppercase and lowercase letters, the number 0 through to 9, punctuation marks, and special control characters used in U.S. English. Most current x86 systems support the use of extended (or “high”) ASCII. Extended ASCII allows the eighth bit of each character to identify an additional 128 special symbol characters, foreign-language letters, and graphic symbols - the ASCII character set.
Despite the fact that ASCII is a "standard", simply encoding data using standard ASCII characters does not guarantee compatibility across systems. While it is true that the letter "A" on one machine is most likely a letter "A" on another machine, there is very little standardisation across machines with respect to the use of the control characters.
In spite of some major shortcomings, ASCII data is the standard for data interchange across computer systems and programs. Most programs can accept ASCII data; likewise, most programs can produce ASCII data.
The rapid growth of the World Wide Web has created a need for a new global standard for software. An international language encoding standard has been created so that computers in one world language community can communicate with those in another language community. This new standard is Unicode – Unicode enabled NTFS uses it to store names of files, directories, and volumes - (the name Unicode implies 'unifying' and that is what it does), a 16-bit standard that is 100% compatible with 7-bit ASCII that depends on code pages being loaded.
In Windows, most internal text strings are stored and processed as 16-bit Unicode characters. Unicode is an international character set standard that defines unique 16-bit values for the world’s known character sets. |
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| Archive Bit |
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The bit in a file’s attribute byte that sets the archive attribute and indicates whether the file has changed since being last backed up. In other words, the archive bit is a binary flag associated with every Windows file; it is always equal to either 1 or 0 for a given file. When a file is first created, the archive bit is set to 1. Likewise, any time a file is later modified, the archive bit is set to 1. The only time the archive bit for a given file is set to 0 is if some program specifically tells Windows to reset it.
The premise for using the archive bit is that backup software can reset the archive bit to 0 when it backs up a given file. Consequently, the next time the backup program runs, it can easily locate the files that are created or modified since the last backup, since these will be the only files whose archive bit is not cleared (1). The limitation of using the archive bit is that other applications, particularly backup programs, can independently reset it. The timestamp or modified time property of a file can be used instead of the archive bit. See: Backup Types (Thou shalt make regular and complete backups). |
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| Areal Density |
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Areal Density, sometimes referred to as “bit density”, is a calculation (the “product”), and the unit of measurement, of the bit density (bits per (square) inch (BPI)), defining how many bits can be written onto one linear inch of a track) multiplied by the track density (tracks per (square) inch (TPI)), defining how many tracks are recorded onto one linear inch of a track), which results in a figure indicating how many bits per square inch are present on the hard disk drives surface. In other words, it is the product of BPI and TPI, which reflects the amount of user data that can be stored reliably in one square unit of area on the surface of a hard disk drive’s platter. It is now measured in gigabits per square inch (Gb/in2) and is the metric used to quantify the impressive growth in hard disk drive data storage capacity.
Data is arranged on a hard disk drive in concentric circles, called tracks. To gain more data on a track, the spacing between each bit in the down-track direction must decrease. The data density in this direction, also called the linear or recording density, is measured in thousands of bits per inch (KBPI). Similarly, the track density across the hard disk drive is measured in thousands of tracks per inch (KTPI) – how close-fitting the concentric tracks are per square inch. The TPI metric not only reflects the width of the written track, but also the small guardbands that are needed between tracks to provide margins for head-to-track misalignment.
Areal density has increased by almost 8 orders-of-magnitude (10^8) since the introduction of the first hard disk drive. For decades, the compound annual growth rate (CAGR) of areal density was about 30%. With the introduction of MR (magneto-resistance) by head technology around 1990-1991, the rate increased to 60%. When GMR (giant magnet-resistance) heads were introduced by IBM in the late 90's, the CAGR temporarily increased to over 100%, during which time there was an increase in the number of companies that exited the industry or merged. The pace of areal density growth is now slowing and should settle somewhere between the historical rates of 30 and 60%. Technological advances, although extremely challenging, to the media layer on the platters, actuators, control electronics and other components, especially in the case of read/write heads, are made to continually improve areal density.
The track-to-track spacing (track pitch) is 8 micro-inches (about 200 nano-meters). The 5–10% guardbands between tracks are a fraction of a micro-inch (less than 20 nano-meters).
Another important aspect pertaining to areal density is the following. The gradual degradation of magnetic information is often referred to as thermal decay. There may be millions of atoms that are magnetically oriented in a certain direction in one bit. Over time, thermal energy, i.e. heat, will make some of the atoms “forget” their magnetic orientation in the bit. As there are fewer atoms maintaining the data, it follows that it takes even less thermal energy to get a few more atoms to forget the data. After some period of time an “avalanche point” is reached resulting in a random orientation for the bit and the data is lost – it has decayed. This is referred to as the super-paramagnetic effect. The thermal stability of bits drops rapidly as the areal density increases – there are fewer atoms in each bit to retain the magnetic orientation. High temperature environments can make thermal decay worse. It has also been demonstrated that writing to a particular track can cause degradation in the bits in the adjacent tracks. It is not widely known, but many modern hard disk drives routinely check for thermal decay of bits in the field and rewrite the sectors in which degradation is identified.
See: Data, and Data Loss & Data Recovery.
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| Attribute (or Attribute Byte) |
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| A byte of information, held in the directory entry of any file or folder that describes various attributes of the file or folder. For example, whether it is read-only, hidden, compressed, encrypted, whether the file’s contents should be indexed for fast file searching or if it has been backed up since it was last changed (ready for archiving). Attributes can be set by the DOS ATTRIB command or within Windows Explorer.See: Attribute (Resident & Non-Resident), and Attribute List. |
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| Attribute (Resident & Non-Resident) |
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Small files and folders (typically 1,500 bytes or smaller) are entirely contained within the file’s master file table (MFT) record including all its attributes and values fit in the file record with the MFT, e.g., data,. When the value of an attribute is stored directly in the MFT, the attribute is called a resident attribute. Several attributes are defined as always being resident so that NTFS can locate non-resident attributes. The standard information and the index root attributes are always resident.
Each attribute begins with a standard header containing information about the attribute, and information that NTFS uses to manage the attributes in a generic manner. The header, which is always resident, records whether the attribute’s value is resident or non-resident. For resident attributes, the header also contains the offset from the header to the attribute’s value and the length of the attribute’s value.
When an attribute’s value is stored directly in the MFT, the time it takes NTFS to access the value is greatly reduced as a consequence of not having to look up the value in the MFT and then reading a succession of allocation units to find the file’s data, which a FAT filesystem does. NTFS accesses the disk once and retrieves the data immediately.
The attribute for a small directory, as well as a small file, can be resident in the MFT. For a small directory or folder (typically 1,500 bytes or smaller), the index root attribute contains an index of the references for the file and the subdirectories in the directory.
Nevertheless, not all files, folders or directories are small enough to fit completely into a 1KB fixed size MFT record. If a particular attribute, such as a file’s data attribute is too large to be contained in an MFT file record. For large files, folders and directories, NTFS allocates clusters for the attribute’s data in separate MFT B-tree (or b-tree) structures that have records with pointers to external clusters that contain folder entries that cannot be contained within the MFT structure. This area is called a run (or an extent). If the attribute’s value later grows, NTFS allocates another run. Attributes whose values are stored in runs rather than in the MFT are called non-resident attributes. The filesystem decides whether a particular attribute is resident or non-resident; the location of the data is transparent to the process accessing it.
When an attribute is non-resident, as the data attribute for a large file may be, its header contains the information NTFS needs to locate the attribute’s value on the disk.
Among the standard attributes, only those that can grow can be non-resident. For a file, the attributes that can grow are the data and the attribute list. The standard information and filename attributes are always resident.
A large directory can also have non-resident attributes (or parts of attributes). The standard information and the filename attributes are always a resident attribute. The header and at least part of the value of the index root attribute are also resident for directories.
When a file, folder or a directory’s attributes cannot fit in the MFT file record and separate allocations are needed, NTFS keeps track of the runs by means of VCN-LCN mapping pairs (each VCN maps to a corresponding, or is associated with LCN, which identifies the disk location of the cluster and in turn enumerates the clusters of a file, for example. LCNs represent the sequence of clusters on the entire volume from zero (0) through to n. VCNs number the clusters belonging to a particular file from zero (0) through to m.
Moreover, if a particular file has too many attributes to fit in the MFT file record, a second MFT file record is used to contain the additional attributes (or attribute headers for non-resident attributes). In such cases, an attribute called the attribute list is added. The attribute list attribute contains the name and type code of each of the file’s attributes and the file reference of the MFT file record where the attributer is located. The attribute list attribute is provided for those cases in which a file grows so large or is so fragmented that a single MFT file record cannot contain the multitude of VCN-to-LCN mappings needed to find all its runs. Files with more than 200 runs typically require an attribute list.
Note: Run list corruption will result in filesystem instability and data lost. Use tools that increase the size of the MFT with caution.
See: Attribute (or Attribute Byte), Attribute List, B-tree (or b-tree), and Master File Table
(MFT). |
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| Attribute List |
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A special kind of file attribute in an NTFS file header that contains additional attributes. The attribute list is created if a particular file has too many attributes to fit in the master file table (MFT) record. The attribute list attribute contains the name and the type code of each of the file’s attributes and the file reference of the MFT record where the attribute is located.
See: Attribute (Resident & Non-Resident), Attribute (or Attribute Byte), B-tree (or b-tree), and Master File Table (MFT). |
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| Automatic Backup Scheduling |
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| Automating common backup tasks can greatly reduce the amount of time and energy that must be spent performing daily backups. |
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