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How NTFS File System Works: NTFS Physical Structure (5)

September 17th, 2009

NTFS File Record Attributes

Every allocated sector on an NTFS volume belongs to a file. Even the file system metadata is part of a file. NTFS views each file (or folder) as a set of file attributes. File elements such as its name, its security information, and even its data are file attributes. Each attribute is identified by an attribute type code and an optional attribute name.

File and folder records are 1 KB each and are stored in the MFT, the attributes of which are written to the allocated space in the MFT. Besides file attributes, each file record contains information about the position of the file record in the MFT.

When a file’s attributes can fit within the MFT file record for that file, they are called resident attributes. Attributes such as file name and time stamp are always resident. When the amount of information for a file does not fit in its MFT file record, some file attributes become nonresident. Nonresident attributes are allocated one or more clusters of disk space. A portion of the nonresident attribute remains in the MFT and points to the external clusters. NTFS creates the Attribute List attribute to describe the location of all attribute records. The table NTFS File Attribute Types lists the file attributes currently defined by NTFS.

NTFS File Attribute Types

Attribute Type Description
Standard Information Information such as access mode (read-only, read/write, and so forth) timestamp, and link count.
Attribute List Locations of all attribute records that do not fit in the MFT record.
File Name A repeatable attribute for both long and short file names. The long name of the file can be up to 255 Unicode characters. The short name is the 8.3, case-insensitive name for the file. Additional names, or hard links, required by POSIX can be included as additional file name attributes.
Data File data. NTFS supports multiple data attributes per file. Each file typically has one unnamed data attribute. A file can also have one or more named data attributes.
Object ID A volume-unique file identifier. Used by the distributed link tracking service. Not all files have object identifiers.
Logged Tool Stream Similar to a data stream, but operations are logged to the NTFS log file just like NTFS metadata changes. This attribute is used by EFS.
Reparse Point Used for mounted drives. This is also used by Installable File System (IFS) filter drivers to mark certain files as special to that driver.
Index Root Used to implement folders and other indexes.
Index Allocation Used to implement the B-tree structure for large folders and other large indexes.
Bitmap Used to implement the B-tree structure for large folders and other large indexes.
Volume Information Used only in the $Volume system file. Contains the volume version.

NTFS creates a file record for each file and a folder record for each folder created on an NTFS volume. The MFT includes a separate file record for the MFT itself. These file and folder records are 1 KB each and are stored in the MFT. The attributes of the file are written to the allocated space in the MFT. Besides file attributes, each file record contains information about the position of the file record in the MFT. The figure MFT Entry with Resident Record shows the contents of an MFT record for a small file or folder. Small files and folders (typically, 900 bytes or smaller) are entirely contained within the file’s MFT record.

MFT Entry with Resident Record

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Typically, each file uses one file record. However, if a file has a large number of attributes or becomes highly fragmented, it might need more than one file record. If this is the case, the first record for the file, the base file record, stores the location of the other file records required by the file.

Folder records contain index information. Small folder records reside entirely within the MFT structure, while large folders are organized B-tree structures and have records with pointers to external clusters that contain folder entries that cannot be contained within the MFT structure.

The benefit of using B-tree structures is evident when NTFS enumerates files in a large folder. The B-tree structure allows NTFS to group, or index, similar file names and then search only the group that contains the file, minimizing the number of disk accesses needed to find a particular file, especially for large folders. Because of the B-tree structure, NTFS outperforms FAT for large folders because FAT must scan all file names in a large folder before listing all of the files.

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