MECH 2301 Lecture Notes - Lecture 34: Free Space Bitmap, Bitstream, Security Descriptor
22 May 2018
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MECH 2301 Lecture 34 Notes
Introduction
Bitmap method
• An attribute is a stream of bytes that describes some aspect of the file.
• Standard attributes include the file name, its security descriptor, time stamps, read-only
and archive flags, links, and data i.e., the file’s contents.
• Directory files have attributes that index the directory.
• Each attribute has a name or number plus a byte stream representing the value of the
attribute.
• The primary data stream is unnamed, but it is possible to have named data streams in
addition.
• Thus, there may be multiple data streams in a single file record. Small files may fit within
the MFT record itself.
• For larger files, the MFT record will contain pointers to clusters in an area of the disk
outside the MFT.
• Attributes that extend beyond the MFT are called nonresident attributes (usually the
data attribute, obviously).
• Nonresident clusters are called runs.
• If the attribute outgrows its space, the file system continues to allocate runs as needed.
• To allocate new blocks as they are required, the file management system must keep a
list of the free available space on a disk.
• To create a new file or add blocks to an existing file, the file manager takes space from
the free space list.
• When a file is deleted, its space is returned to the free space list.
• There are two methods commonly used for this purpose.
• One method of maintaining a free space list is to provide a table with one bit for each
block on the disk.
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