Hard Drives

Hard Drives

A hard drive is a series of magnetic disks that 'in essence' store a series of 1's and 0's that translate into different types of data.

A hard disk will be constantly spinning whilst the device is turned on, at a rate of around 7200 revolutions per minute. Obviously this is going to create a lot of heat so the cooling of the hard disc is crucial.

Properties of a hard drive include:

• its Capacity
• its expected lifespan
• the drives access time
• the drives seek time
• the drives Latency

The capacity of a disc is calculated by the following:

(No. bytes per sector) x (no. sectors per track) x (no. tracks per cylinder) x (no. cylinders per disc)

A hard drives lifespan is referred to as its MTBF, or 'mean time before fail'.

The drives access time is derived from - Seek time + latency.

The drives seek time is the time it takes to locate the required cylinder on the disk.

The drives latency is the time it takes for the arm to move to the required cylinder on the disk.

An alternate hard drive to the disk drive is becoming more and more popular, the Solid State Disk (SSD).

Solid State Disk

A solid state disk has no moving parts within which has certain advantages on its predecessor. 

• No moving parts mean less likely that the disc is damaged when being moved.
• The seek time is reduced.
• Less latency
• Quieter with no noise at all.

The SSD is more expensive though.

Servers

A server needs a good architecture so that when a failure occurs, all the information on that server is not lost.

RAID (redundant array of inexpensive disks) is a data storage facility that uses multiple hard drives logically to eliminate the risk of data loss upon failure. A raid system has to have at least one RAID controller.

There are multiple RAID architectures that are named from 0 to 5.

RAID 0:

RAID 1:

From this architecture you can see that drive 1 (split into 6 blocks) is basically mirrored into drive 2, meaning that if one disk fails, the back up is still there with all the data. This was the case in the twin towers, one drive in each tower. But obviously as both towers went down, all the data was lost. Now companies put each disk in different geographical locations.

RAID 0 & 1

This architecture takes RAID 1 & 0 and utilises both.

There are RAIDS 2,3,4 and 5 but they aren't generally used.

Storing on a CD

• Can hold up to 800 MB

Storing on a DVD

• Can hold up to 17 GB

Storing on Blu-Ray

• Can be dual layer, 25 GB per layer