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  Physical structure and process disk IO
     
  Add Date : 2018-11-21      
         
       
         
  For manage disks, disk sub-surface, head, track, cylinder, and sector.

    Disk surface: The disk is composed of a stack of disk surface, below left.

    Head (Heads): one for each head disk surface, is responsible for reading and writing data to the disk surface.

    Track (Track): Each disk will be divided into a plurality of concentric circles around the center circle, each circle is called a track.

    Cylinders (Cylinders): All the same three-dimensional position of the tracks on the disc is called a cylinder composed.

    Sector (Sector): to manage disk units of track is still too large, the computer predecessors again each track divided into a plurality of sectors

On Linux you can fdisk command to view the information systems use these physical disks present.
[Root @ dbserver ~] # fdisk -l

 Disk / dev / sda: 1199.6 GB, 1199638052864 bytes
 255 heads, 63 sectors / track, 145847 cylinders
 Units = cylinders of 16065 * 512 = 8225280 bytes
 Sector size (logical / physical): 512 bytes / 512 bytes
 I / O size (minimum / optimal): 512 bytes / 512 bytes
 Disk identifier: 0x54ab02ca

    Device Boot Start End Blocks Id System
/ Dev / sda1 1 5 40131 de Dell Utility
 Partition 1 does not end on cylinder boundary.
 / Dev / sda2 * 6 267 2097152 c W95 FAT32 (LBA)
 ......

 
I can see the disk has 255 heads, that is to say a total of 255 disk. 145 847 cylinders, that is to say on each disk has 145,847 tracks, 63sectors / track talking about a total of 63 sectors on each track. The results are also given command value Sector size is 512bytes. Then we put pen to paper to count the size of the disk.
Cylindrical disk 255 * 145 847 * * 63 sectors each sector 512bytes = 1199632412160 byte = 1117.25GB
As a result 1117.25GB, and the total size of the disk matches.
 In the above chart can be found in an error (marked red), start position, that is / dev / sda1 from the beginning of the first sector erasures, if the first 0-62 sectors, namely, the first track (cylinders) left alone disk MBR and from 64 sectors, namely the second tracks (cylinders) start partition will be the performance of the file system will bring greatly improved.
  
Let us look at the process of disk IO time.
    The first step is to find the radial movement of the head track data is located. This portion is called seek time.
    The second step, to find the target track after the adoption of the disk rotation will move to the target sector just below the head.
    The third step is to read or write data to the target sector. So far, a single disk IO completion.
 Therefore: a single disk IO time = seek time + rotational delay + access time.

 For rotational latencies, and now the mainstream server is often used is 1W rev / min disk, the time required for each revolution of 60 * 1000/10000 = 6ms, so the rotational delay (0-6ms). For access time, generally time-consuming short, a few tenths of ms. For the seek time, probably in modern disk 3-15ms, which seek mainly influenced by the size of the current position of the head where the track location and relative distance of the target.
 Operating system by pressing the corresponding track cylinder zoning to reduce seek time of disk IO spent, thus improving read and write performance of the disk.
     
         
       
         
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