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FILE STORAGE HARDWARE AND DISK ORGANIZATION

                                      

Hard Disk Drive Basics

A hard disk is a sealed unit containing a number of platters in a stack. Hard disks may be mounted in a horizontal or a vertical position. In this description, the hard drive is mounted horizontally.

Electromagnetic read/write heads are positioned above and below each platter. As the platters spin, the drive heads move in toward the center surface and out toward the edge. In this way, the drive heads can reach the entire surface of each platter.

 

Making Tracks

On a hard disk, data is stored in thin, concentric bands. A drive head, while in one position can read or write a circular ring, or band called a track. There can be more than a thousand tracks on a 3.5-inch hard disk. Sections within each track are called sectors. A sector is the smallest physical storage unit on a disk, and is almost always 512 bytes (0.5 kB) in size.

The figure below shows a hard disk with two platters.

Figure 3-1 Parts of a Hard Drive

The structure of older hard drives (i.e. prior to Windows 95) will refer to a cylinder/ head/ sector notation. A cylinder is formed while all drive heads are in the same position on the disk. The tracks, stacked on top of each other form a cylinder. This scheme is slowly being eliminated with modern hard drives. All new disks use a translation factor to make their actual hardware layout appear continuous, as this is the way that operating systems from Windows 95 onward like to work.

To the operating system of a computer, tracks are logical rather than physical in structure, and are established when the disk is low-level formatted. Tracks are numbered, starting at 0 (the outermost edge of the disk), and going up to the highest numbered track, typically 1023, (close to the center). Similarly, there are 1,024 cylinders (numbered from 0 to 1023) on a hard disk.

The stack of platters rotate at a constant speed. The drive head, while positioned close to the center of the disk reads from a surface that is passing by more slowly than the surface at the outer edges of the disk. To compensate for this physical difference, tracks near the outside of the disk are less-densely populated with data than the tracks near the center of the disk. The result of the different data density is that the same amount of data can be read over the same period of time, from any drive head position.

The disk space is filled with data according to a standard plan. One side of one platter contains space reserved for hardware track-positioning information and is not available to the operating system. Thus, a disk assembly containing two platters has three sides available for data. Track-positioning data is written to the disk during assembly at the factory. The system disk controller reads this data to place the drive heads in the correct sector position.

Up

Sectors and Clusters

A sector, being the smallest physical storage unit on the disk, is almost always 512 bytes in size because 512 is a power of 2 (2 to the power of 9). The number 2 is used because there are two states in the most basic of computer languages - on and off.

Each disk sector is labelled using the factory track-positioning data. Sector identification data is written to the area immediately before the contents of the sector and identifies the starting address of the sector.

The optimal method of storing a file on a disk is in a contiguous series, i.e. all data in a stream stored end-to-end in a single line. As many files are larger than 512 bytes, it is up to the file system to allocate sectors to store the file’s data. For example, if the file size is 800 bytes, two 512 k sectors are allocated for the file.

A cluster can consist of one or more consecutive sectors. The number of sectors is always an exponent of 2. A cluster could consist of 1 sector (2^0), or, more frequently, 8 sectors (2^3). The only odd number a of sectors a cluster could consist of is 1. It could not be 5 sectors or an even number that is not an exponent of 2. It would not be 10 sectors, but could be 8 or 16 sectors.

They are called clusters because the space is reserved for the data contents. This process protects the stored data from being over-written. Later, if data is appended to the file and its size grows to 1600 bytes, another two clusters are allocated, storing the entire file within four clusters.

Figure 3-2 Sectors and Clusters

 

If contiguous clusters are not available (clusters that are adjacent to each other on the disk), the second two clusters may be written elsewhere on the same disk or within the same cylinder or on a different cylinder - wherever the file system finds two sectors available. A file stored in this non-contiguous manner is considered to be fragmented. Fragmentation can slow down system performance if the file system must direct the drive heads to several different addresses to find all the data in the file you want to read. The extra time for the heads to travel to a number of addresses causes a delay before the entire file is retrieved.

Cluster size can be changed to optimize file storage. A larger cluster size reduces the potential for fragmentation, but increases the likelihood that clusters will have unused space. Using clusters larger than one sector reduces fragmentation, and reduces the amount of disk space needed to store the information about the used and unused areas on the disk.

Most disks used in personal computers today rotate at a constant angular velocity. The tracks near the outside of the disk are less densely populated with data than the tracks near the center of the disk. Thus, a fixed amount of data can be read in a constant period of time, even though the speed of the disk surface is faster on the tracks located further away from the center of the disk.

Modern disks reserve one side of one platter for track positioning information, which is written to the disk at the factory during disk assembly. It is not available to the operating system. The disk controller uses this information to fine tune the head locations when the heads move to another location on the disk. When a side contains the track position information, that side cannot be used for data. Thus, a disk assembly containing two platters has three sides that are available for data.

Master Boot Record (MBR) 

The Master Boot Record, created when you create the first partition on the hard disk, is probably the most important data structure on the disk. It is the first sector on every disk. The location is always track (cylinder) 0, side (head) 0, and sector 1.

The Master Boot Record contains the Partition Table for the disk and a small amount of executable code. On x86-based computers, the executable code examines the Partition Table, and identifies the system partition. The Master Boot Record then finds the system partition's starting location on the disk, and loads an copy of its Partition Boot Sector into memory. The Master Boot Record then transfers execution to executable code in the Partition Boot Sector.

Note: Although there is a Master Boot Record on every hard disk, the executable code in the sector is used only if the disk is connected to an x86-based computer and the disk contains the system partition.

The example below shows a hex dump of the sector containing the Master Boot Record. The figure shows the sector in two parts:

• The first part is the Master Boot Record, which occupies the first 446 bytes of the sector. The disk signature (FD 4E F2 14) is at the end of the Master Boot Record code.
• The second part is the Partition Table.

  Physical Sector:Cyl 0,Side 0,Sector 1  
  
  00000000:00 33 C0 8E D0 BC 00 7C -8B F4 50 07 50 1F FB FC .3.....|..P.P..  
  00000010:BF 00 06 B9 00 01 F2 A5 -EA 1D 06 00 00 BE BE 07 ................  
  00000020:B3 04 80 3C 80 74 0E 80 -3C 00 75 1C 83 C6 10 FE ...<.t..<.u.....  
  00000030:CB 75 EF CD 18 8B 14 8B -4C 02 8B EE 83 C6 10 FE .u......L.......  
  00000040:CB 74 1A 80 3C 00 74 F4 -BE 8B 06 AC 3C 00 74 0B .t..<.t.....<.t.  
  00000050:56 BB 07 00 B4 0E CD 10 -5E EB F0 EB FE BF 05 00 V.......^.......  
  00000060:BB 00 7C B8 01 02 57 CD -13 5F 73 0C 33 C0 CD 13 ..|...W.._s.3...  
  00000070:4F 75 ED BE A3 06 EB D3 -BE C2 06 BF FE 7D 81 3D Ou...........}.=  
  00000080:55 AA 75 C7 8B F5 EA 00 -7C 00 00 49 6E 76 61 6C U.u.....|..Inval  
  00000090:69 64 20 70 61 72 74 69 -74 69 6F 6E 20 74 61 62 id partition tab  
  000000A0:6C 65 00 45 72 72 6F 72 -20 6C 6F 61 64 69 6E 67 le.Error loading  
  000000B0:20 6F 70 65 72 61 74 69 -6E 67 20 73 79 73 74 65 operating syste  
  000000C0:6D 00 4D 69 73 73 69 6E -67 20 6F 70 65 72 61 74 m.Missing operat  
  000000D0:69 6E 67 20 73 79 73 74 -65 6D 00 00 80 45 14 15 ing system...E..  
  000000E0:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  000000F0:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000100:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000110:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000120:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000130:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000140:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000150:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000160:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000170:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000180:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  00000190:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  000001A0:00 00 00 00 00 00 00 00 -00 00 00 00 00 00 00 00 ................  
  000001B0:00 00 00 00 00 00 00 00 -FD 4E F2 14 00 00       .........N......  
                                                      80 01               ..  
  000001C0:01 00 06 0F 7F 96 3F 00 -00 00 51 42 06 00 00 00 .....?...QB....  
  000001D0:41 97 07 0F FF 2C 90 42 -06 00 A0 3E 06 00 00 00 A....,.B...>....  
  000001E0:C1 2D 05 0F FF 92 30 81 -0C 00 A0 91 01 00 00 00 .-....0......... 
  000001F0:C1 93 01 0F FF A6 D0 12 -0E 00 C0 4E 00 00 55 AA ...........N..U.

Hard Drive Partition. Partition Table.

The information about primary partitions and an extended partition is contained in the Partition Table, a 64-byte data structure located in the same sector as the Master Boot Record (cylinder 0, head 0, sector 1). The Partition Table conforms to a standard layout that is independent of the operating system. Each Partition Table entry is 16 bytes long, making a maximum of four entries available. Each entry starts at a predetermined offset from the beginning of the sector, as follows:

• Partition 1 0x01BE (446)
• Partition 2 0x01CE (462)
• Partition 3 0x01DE (478)
• Partition 4 0x01EE (494)

The last two bytes in the sector are a signature word for the sector and are always 0x55AA. The next example is a printout of the Partition Table for the disk shown in an example earlier in this chapter. When there are fewer than four partitions, the remaining fields are all zeros.

                                                     80 01              .. 
 000001C0:01 00 06 0F 7F 96 3F 00 -00 00 51 42 06 00 00 00 .....?...QB....
 000001D0:41 97 07 0F FF 2C 90 42 -06 00 A0 3E 06 00 00 00 A....,.B...>....
 000001E0:C1 2D 05 0F FF 92 30 81 -0C 00 A0 91 01 00 00 00 .-....0......... 
 000001F0:C1 93 01 0F FF A6 D0 12 -0E 00 C0 4E 00 00 55 AA ...........N..U. 

The following table describes each entry in the Partition Table. The sample values correspond to the information for partition 1.

Partition Table Fields

Byte Offset	Field Length	Sample Value	Meaning
00	BYTE	0x80	Boot Indicator. Indicates whether the partition is the system partition. Legal values are: 00 = Do not use for booting. 80 = System partition.
01	BYTE	0x01	Starting Head.
02	6 bits	0x01	Starting Sector. Only bits 0-5 are used. Bits 6-7 are the upper two bits for the Starting Cylinder field.
03	10 bits	0x00	Starting Cylinder. This field contains the lower 8 bits of the cylinder value. Starting cylinder is thus a 10-bit number, with a maximum value of 1023.
04	BYTE	0x06	System ID. This byte defines the volume type. In Windows NT, it also indicates that a partition is part of a volume that requires the use of the HKEY_LOCAL_MACHINE\SYSTEM\DISK Registry subkey.
05	BYTE	0x0F	Ending Head.
06	6 bits	0x3F	Ending Sector. Only bits 0-5 are used. Bits 6-7 are the upper two bits for the Ending Cylinder field.
07	10 bits	0x196	Ending Cylinder. This field contains the lower 8 bits of the cylinder value. Ending cylinder is thus a 10-bit number, with a maximum value of 1023.
08	DWORD	3F 00 00 00	Relative Sector.
12	DWORD	51 42 06 00	Total Sectors.

The remainder of this section describes the uses of these fields. Definitions of the fields in the Partition Table is the same for primary partitions, extended partitions, and logical drives in extended partitions.

Boot Indicator Field

The Boot Indicator field indicates whether the volume is the system partition. On x-86-based computers, only one primary partition on the disk should have this field set. This field is used only on x86-based computers. On RISC-based computers, the NVRAM contains the information for finding the files to load.

On x86-based computers, it is possible to have different operating systems and different file systems on different volumes. For example, a computer could have MS-DOS on the first primary partition and Windows 95, UNIX, OS/2, or Windows NT on the second. You control which primary partition (active partition in FDISK) to use to start the computer by setting the Boot Indicator field for that partition in the Partition Table.

System ID Field

For primary partitions and logical drives, the System ID field describes the file system used to format the volume. Windows NT uses this field to determine what file system device drivers to load during startup. It also identifies the extended partition, if there is one defined.

These are the values for the System ID field:

Table 3-1 System ID Field Values

Figure presented earlier in this section, has examples of a BIGDOS FAT partition, an NTFS partition, an extended partition, and a 12-bit FAT partition.

If you install Windows NT on a computer that has Windows 95 preinstalled, the FAT partitions might be shown as unknown. If you want to be able to use these partitions when running Windows NT, your only option is to delete the partitions.

OEM versions of Windows 95 support the following four partition types for FAT file systems that Windows NT cannot recognize.

Table 3-2 Partition Types

When you create a volume set or a stripe set, Disk Administrator sets the high bit of the System ID field for each primary partition or logical drive that is a member of the volume. For example, a FAT primary partition or logical drive that is a member of a volume set or a stripe set has a System ID value of 0x86. An NTFS primary partition or logical drive has a System ID value of 0x87. This bit indicates that Windows NT needs to use the HKEY_LOCAL_MACHINE\SYSTEM\DISK Registry subkey to determine how the members of the volume set or stripe set relate to each other. Volumes that have the high bit set can only be accessed by Windows NT.

When a primary partition or logical drive that is a member of a volume set or a stripe set has failed due to write errors or cannot be accessed, the second most significant bit is set. The System ID byte is set to C6 in the case of a FAT volume, or C7 in the case of an NTFS volume.

Note

If you start up MS-DOS, it can only access primary partitions or logical drives that have a value of 0x01, 0x04, 0x05, or 0x06 for the System ID. However, you should be able to delete volumes that have the other values. If you use a MS-DOS-based low-level disk editor, you can read and write any sector, including ones that are in NTFS volumes.

On Windows NT Server, mirror sets and stripe sets with parity also require the use of the Registry subkey HKEY_LOCAL_MACHINE\SYSTEM\DISK to determine how to access the disks.

【作者: 东方之子】【访问统计:】【2007年02月26日 星期一 15:22】【注册】【打印】