Tuesday, May 26, 2015

Physical Disk Structure

Disk Structure:

                          Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer the 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially – Sector 0 is the first sector of the first track on the outermost cylinder – Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.

Physical Disk Structure
Cylinder 
The disc that makes up the hard disk is divided into tracks; tracks of all discs which have same track value are called a cylinder, so the cylinder is a pile of tracks with same track value of a hard disk. 
Head 
Normally, a disc has two heads for reading or writing data, one is for the top and the other one is for the opposite side; the head value means the disc location and side. 

Sector 
A track is composed of sectors and the number of sectors of all tracks on the hard disk is the same.Sector is the minimal storage unit of a hard disk; the size of one sector is always 512 bytes (rarely, it might be 1024, 2048 or 4096 bytes in some special hard disks). 

Components of Disk Drive

Components of Disk Drive are 
   
  •         Platter and Read/Write Head
  •         HAD
  •         Power Connectors
  •         Interface
  •         Controller Board

Components of Disk Drive




Storage Options in Data Center

      Magnetic Tape

4  Low cost solution for long term data storage
Ø  Preferred option for backup destination in the past

4  Limitations
Ø  Sequential data access
Ø  Single application access at a time
Ø  Physical wear and tear
Ø  Storage/retrieval overheads

      Optical discs

4  Popularly used as distribution medium in small, single-user computing environments
4  Limited in capacity and speed
4  Write once and read many (WORM): CD-ROM, DVD-ROM
4  Other variations: CD-RW, Blu-ray discs

      Disk drive

4  Most popular storage medium
4  Large storage capacity
4  Random read/write access

      Flash drives

4  Uses semiconductor media
4  Provide high performance and low power consumption

Fibre Channel and Internet Protocol

      Fibre Channel (FC)

4  Widely used protocol for high speed communication to the storage device

4  Provides a serial data transmission that operates over copper wire and/or optical fiber
4  Latest version of the FC interface ‘16FC’ allows transmission of data up to 16 Gb/s

      Internet Protocol (IP)

4  Traditionally used to transfer host-to-host traffic

4  Provide opportunity to leverage existing IP based network for storage communication

Ø  Examples: iSCSI and FCIP protocols

SCSI and SAS

          Parallel Small computer system interface (SCSI)


4  Popular standard for connecting host and peripheral devices

Ø  Commonly used for storage connectivity in servers

4  Higher cost than IDE/ATA, therefore not popular in PC environments

4  Available in wide variety of related technologies and standards

4  Support up to 16 devices on a single bus

4  Ultra-640 version provides data transfer speed up to 640 MB/s

      Serial Attached SCSI (SAS)


4  Point-to-point serial protocol replacing parallel SCSI

4  Supports data transfer rate up to 6 Gb/s (SAS 2.0)


IDE/ATA and Serial ATA

      Integrated Device Electronics (IDE)/Advanced Technology Attachment (ATA)

4  Popular interface used to connect hard disks or CD-ROM drives

4  Available with variety of standards and names

      Serial Advanced Technology Attachment (SATA)

4  Serial version of the IDE/ATA specification that has replaced the parallel ATA

4  Inexpensive storage interconnect, typically used for internal connectivity


4  Provides data transfer rate up to 6 Gb/s (standard 3.0)


Connectivity in DataCenter

Connectivity :
 

      Interconnection between hosts or between a host and peripheral devices, such as storage 

      Physical Components of Connectivity are:
4  Host interface card, port, and cable

      Protocol = a defined format for communication between sending and receiving devices
4  Popular storage interface protocols: IDE/ATA and SCSI