Week 6 - HDD Structure (the PHYSICAL Structure of Disks) Flashcards
Name the key components of a HDD
PLATTERS
READ WRITE HEADS
SPINDLE
ACTUATOR COLLUMN
CONTROLLER
PCB (Printed Circuit Board)
DATA CONNECTION
POWER CONNECTION
What does the ACTUATOR COLLUMN DO?
One single actuator collumn powers the platters, read write heads and spindle - so they cannot work independently of each other and remain perfectly alligned.
What is the SPINDLE?
The spindle (goes down the centre of the platters) is what holds the hard drive’s platters in place. With a traditional hard drive, there is the need to have multiple platters. The spindle holds these platters in a fixed position with enough space for the read/write arms to get to the data on the disks.
What are the PLATTERS?
The platters are where the file information is stored. The use magnatism to record info.
There are usually between 4 to 8 platters inside a disk.
The disk is organised into:
TRACKS (concentric circles - go all way round like lanes on a running track)
GEOMETRIC SECTOR (a wedge of cake)
SECTORS (the smallest addressible unit on a HDD. Every sector on an HDD is same size (usually 512 bytes). It is the bit of track that exists inside the geometric secor.
(Actually bigger then 512 (usually about 571 bytes) but the OS only sees that data area of 512 bytes. The data area is 512 bytes - there is also a sync area, header and CRC value).
CLUSTERS - not strictly part of the HDD structure but because sectors are usually too small for the file system to use, they get grouped together into clusters which helps the file system organise them in a more structured way. Live files cannot share clusters - only one live file per cluster. Files can use as many clusters as they need.
In some File Systems, clusters are called BLOCKS.
What are the READ WRITE HEADS?
There is a read write head for each side of every platter (2 per platter).
They hover above the platter at a height of only 3nm (smaller than a smoke particle) - this is why HDD have to be well sealed.
If the read write head makes contact with the platter then it causes a HEAD CRASH - a catestrophic common HDD failure.
The read write head must be in position over the correct place to read data. It moves in and out to the correct track, then waits until the correct sector is under it.
Time it takes to get to the correct track is SEEK TIME (usually between 3 - 15 ms). Time it waits for the correct sector to be below the head is the ROTANTIONAL DELAY or LATENCY.
The sum of thrse two is how long it takes to access the required info
What is the CONTROLLER in a HDD?
The hard disk controller, known as the adapter of hard disk driver, is the interface device between computers and hard disk driver.
What is the PCB (Printed Circuit Board)?
PCB is a critical component of a HDD, typically located at the bottom or side, carrying various components such as control chips, BIOS chips, power managers, etc
How do we find the data we need on a HDD?
Previously used CHS (Cylinder (or track) head and sector. This sytem specified the read write head to use, the track number and the sector. Complicated and did not scale well.
Now use LBA - Logical Block Addressing
A linear index scheme: first block is LBA1, second is :LBA1 etc.
Usually 48 bits used in addressing so 2to the power of 48 blocks can be addressed.
There are basically 3 types of HDD that use this platter / read write head etc structure. Describe these 3 types
The three are distinguished by their connectors, their motherboard and speed.
HDD speed is measured in RPM (no. of times the platter spins in 1 minute or rotational speed)
IDE - Integrated Drive Electonics (think ribbon connectors). 4,800 to 7,200 RPM
SATA (Serial Advanced Technology)
7,200 to 10,000 RPM
Small Computer System Interface (SCSI) ‘Scuzzy’. 15,000 RPM
In relation to speed the seek time also needs to be taken into account (time it takes for the read / write head to position itself over the correct track). Values are usually between 3 to 15 ms
