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Mass Storage Structure (Disk Structure (Constant angular velocity (The…
Mass Storage Structure
Disk Structure
Modern magnetic disk drives are addressed as large one dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.
The size of a logical block is usually 512 bytes, although some disks can be low-level formatted to have a different logical block size, such as 1,024 bytes.
On media that use constant linear velocity (CLV), the density of bits per track is uniform. The farther a track is from the center of the disk, the greater its length, so the more sectors it can hold.
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Disk Attachment
Host-Attached Storage
Host-attached storage is storage accessed through local I/O ports. These ports use several technologies.
A wide variety of storage devices are suitable for use as host-attached storage. Among these are hard disk drives, RAID arrays, and CD, DVD, and tape drives.
The I/O commands that initiate data transfers to a host-attached storage device are reads and writes of logical data blocks directed to specifically identified storage units.
Network-Attached Storage
A network-attached storage (NAS) device is a special-purpose storage system that is accessed remotely over a data network
Clients access network-attached storage via a remote-procedure-call interface such as NFS for UNIX systems or CIFS for Windows machines.
Network-attached storage provides a convenient way for all the computers on a LAN to share a pool of storage with the same ease of naming and access enjoyed with local host-attached storage.
Disk Scheduling
FCFS Scheduling
The simplest form of disk scheduling. This algorithm is naturally fair, but it generally does not provide the fastest service.
SSTF Scheduling
It service all the requests close to the current head position before moving the head far away to service other requests. Selects the request with the least seek time from the current head position.
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C-SCAN Scheduling
Variant of SCAN designed to provide a more uniform wait time. It moves the head from one end of the disk to the other, servicing requests along the way.
SCAN Scheduling
The disk arm starts at one end of the disk and moves toward the other end, servicing requests as it reaches each cylinder, until it gets to the other end of the disk.
The SCAN algorithm is sometimes called the elevator algorithm, since the disk arm behaves just like an elevator in a building.
Disk Management
Disk Formatting
A new magnetic disk is a blank slate: it is just a platter 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.
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, and a trailer.
Before it can 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.
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The second step is logical formatting, or creation of a file system.
Boot Block
For a computer to start running it must have an initial program to run called bootstrap. This bootstrap It initializes all aspects of the system
The bootstrap is stored in ROM. This location is convenient, because ROM needs no initialization and is at a fixed location that the processor can start executing when powered up
The full bootstrap program is stored in the “boot blocks” at a fixed location on the disk. A disk that has a boot partition is called a boot disk or system disk.