Page 251 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes 7.5 Disk Management

The operating system is responsible for several other aspects of disk management, too. Here we
discuss disk initialization, booting from disk, and bad-block recovery.
7.5.1 Disk Formatting

A new magnetic disk is a blank slate: It is just platters of a magnetic recording material. Before
a disk can store data, it must be divided into sectors that the disk controller can read and write.
This process is called low-level formatting (or physical formatting). Low-level formatting fills
the disk with a special data structure for each sector. The data structure for a sector typically
consists of a header, a data area (usually 512 bytes in size), and a trailer. The header and trailer
contain information used by the disk controller, such as a sector number and an error-correcting
code (ECC). When the controller writes a sector of data during normal I/O, the ECC is updated
with a value calculated from all the bytes in the data area. When the sector is read, the ECC is
recalculated and is compared with the stored value. If the stored and calculated numbers are
different, this mismatch indicates that the data area of the sector has become corrupted and that
the disk sector may be bad. The ECC is an error-correcting code because it contains enough
information that, if only a few bits of data have been corrupted, the controller can identify
which bits have changed and can calculate what their correct values should be. The controller
automatically does the ECC processing whenever a sector is read or written. Most hard disks are
low-level formatted at the factory as a part of the manufacturing process. This formatting enables
the manufacturer to test the Idisk and to initialize the mapping from logical block numbers to
defect-free sectors on the disk. For many hard disks, when the disk controller is instructed to
low-level format the disk, it can also be told how many bytes of data space to leave between
the header and trailer of all sectors. It is usually possible to choose among a few sizes, such as
256, 512, and 1,024 bytes. Formatting a disk with a larger sector size means that fewer sectors
can fit on each track, but that also means fewer headers and trailers are written on each track,
and thus increases the space available for user data. Some operating systems can handle only
a sector size of 512 bytes.

To use a disk to hold files, the operating system still needs to record its own data structures on
the disk. It does so in two steps. The first step is to partition the disk into one or more groups
of cylinders. The operating system can treat each partition as though it were a separate disk.
For instance, one partition can hold a copy of the operating system’s executable code, while
another holds user files. After partitioning, the second step is logical formatting (or creation
of a file system). In this step, the operating system stores the initial file-system data structures
onto the disk. These data structures may include maps of free and allocated space (a FAT or
inodes) and an initial empty directory.
Some operating systems give special programs the ability to use a disk partition as a large
sequential array of logical blocks, without any file-system data structures. This array is sometimes
called the raw disk, and I/O to this array is termed raw I/O. For example, some database
systems prefer raw I/O because it enables them to control the exact disk location where each
database record is stored. Raw 1/0 bypasses all the file-system services, such as the buffer cache,
file locking, prefetching, space allocation, file names, and directories. We can make certain
applications more efficient by implementing their own special-purpose storage services on a raw
partition, but most applications perform better when they use the regular file-system services.

Give the step of disk management.

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