One disk dies on a Monday
A small firm keeps all its work on one big disk. On Monday morning, the disk makes a clicking noise and stops. Three years of work is gone.
A firm next door has four disks working together. One of its disks also dies on Monday. Nobody notices until an alert arrives. The work carries on.
What is the difference? A way of joining disks called RAID.
Key words
| Word | What it means |
|---|---|
| RAID | a way of joining several disks so they act as one |
| Striping | splitting data into pieces and spreading them across disks |
| Mirroring | writing the same data to two or more disks |
| Parity | an extra check number that lets a lost piece of data be worked out |
| Fault tolerance | carrying on working after a part fails |
| Usable space | the space left for your data after the disks have kept some for safety |
| Database | an organised store of data that programs can search |
| Backup | a separate copy of data kept so you can restore it later |
| Redundancy | having a spare part ready, in case the first one fails |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can explain RAID 0, 1, 5 and 10, work out the usable space for each, and say how many disks can fail.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
RAID 0 and RAID 1
RAID 0 uses striping. Data is split in pieces and spread over the disks. All the disks work at once, so it is fast. But there is no spare. If one disk fails, all the data is lost.
RAID 1 uses mirroring. Every disk holds the same data. If one disk fails, another has the lot. It is safe, but you pay for two disks and get the space of one.
RAID 5 and parity
RAID 5 spreads data and parity over at least three disks. Parity is an extra check number. If one disk fails, the system works the lost piece out from the others.
One disk can fail and nothing is lost. You give up the space of one disk for the parity. A second failure at the same time loses everything.
The table below uses addition to show the idea. Real RAID 5 uses a different sum on bits, called XOR, but the idea is the same.
Worked example: parity with a simple sum
| Disk A holds | Disk B holds | Parity disk holds | Disk B fails. Work out B |
|---|---|---|---|
| 3 | 5 | 8 (3 plus 5) | 8 take away 3 is 5 |
| 7 | 2 | 9 (7 plus 2) | 9 take away 7 is 2 |
The cells students fill in are left blank in this preview.
Quick check
Have a go
Quick check
4 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.