“More than you can count, detective.”
“I’m going to ask you again. If I don’t get the right answer this time I’m going to shut you down.”
“Okay, please, don’t shut me down or restart me. There are many RAID levels, really.”
“I want to take it easy today.”
“Say no more. I think you should check out RAID 2, 3 and 4. You could finish by lunch, no sweat.”
The detective wasn’t convinced by the OS’ answer but what choice did he have? He trudged all the way to RAID 2.
He wasn’t prepared for what he saw next. RAIDs 2, 3 and 4 all looked the same from the outside!
This is how data flows:
We’ve seen earlier that data is written in blocks. However, there are situations where you’d want data to be written in bits (the smallest possible) and bytes (8 bits, the standard word size).
RAID 2 lets you write in bits. In fact, it lets you stripe in bits – each subsequent bit is written on a separate drive. Then, to keep its redundancy tag intact, it adds parity.
Where are the parity bits written?
On a separate drive. This is why RAID 2 is called bit-level striping with dedicated parity. Parity data is on a dedicated drive.
RAID 3 has byte-level striping with dedicated parity. Like RAID 2, RAID 3 also has a dedicated parity drive, and parity isn’t mixed with the main information.
You know what’s coming next.
RAID 4 is block-level striping with dedicated parity. All parity data is written to a single drive.
Because the parity drive is written to a single drive the overall write performance will depend directly on how good the write performance of this drive is. This could cause bottlenecks.
This is the main complication with parity anyway. Parity data needs to be calculated while new data is being added. If a drive fails, the information is rebuilt again. These calculations are taken care of by the controller, but it adds to the overhead.
RAID 2 and RAID 3 are almost extinct. RAID 4 is very rare. Why?
Because there is a way to spread the parity data to multiple disks, eliminating many of the bottleneck problems associated with RAIDs 2, 3 and 4. Good to know neither of these were involved in the ‘murder’.
Lunch time! The detective though it was best to put it off until tomorrow. At least he was ahead of the curve, and in the home stretch.
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