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What is RAID?

A technology that combines several disks and uses them like a single storage device to improve speed or reliability. Its levels include RAID 0 (striping, which splits data across disks), RAID 1 (mirroring, which writes the same data to two disks), RAID 5 and RAID 6 (which write parity distributed across the disks), and RAID 10 (which combines mirroring and striping).

Japanese name: RAID

RAID appears in 5 practice questions across 1 topic, including System Architecture and Reliability.

Field: System Architecture and Reliability

Main RAID levels and capacity

With n disks of the same capacity, the capacity available for data is n disks' worth for RAID 0, half for RAID 1 and RAID 10, n − 1 disks' worth for RAID 5, and n − 2 disks' worth for RAID 6. For example, with four 4 TB disks, RAID 0 gives 16 TB, RAID 10 gives 8 TB, RAID 5 gives 12 TB and RAID 6 gives 8 TB. RAID 0 has no redundancy and loses data when one disk fails; RAID 5 tolerates one failure and RAID 6 any two. RAID 10 tolerates two or more failures as long as both disks of the same mirrored pair do not fail.

Write performance and rebuilds

In RAID 5, each small write reads the old data and old parity and writes the new data and new parity, so one write needs four I/O operations (the write penalty). It is six for RAID 6 and two for RAID 1 and RAID 10. When a failed disk is replaced, a rebuild reconstructs its contents from the data on the remaining disks, but the larger the disks, the longer it takes, and if another disk fails during that time, RAID 5 loses data. For this reason RAID 6 and similar levels are often chosen for large disks.

RAID is not a backup

RAID is a mechanism for continuing to process even when a disk fails. If data is deleted by mistake or encrypted by ransomware, the change is reflected on all disks and cannot be undone. It also does not protect against failure of the RAID device itself or disasters such as fire. Protecting data requires backups to other media or other locations, separately from RAID.

Try it: questions about RAID

Which describes RAID 1?

  1. AData is divided and written across several disks (striping) to speed up reads and writes, but there is no redundancy
  2. BThe same data is written to two disks (mirroring), so no data is lost if one disk fails
  3. CData and parity for error correction are written distributed across several disks, so data can be restored if one disk fails
Show answer and explanation

Answer: B. The same data is written to two disks (mirroring), so no data is lost if one disk fails

RAID 1 is mirroring, which writes the same data to two disks, so if one disk fails, processing can continue with the data on the other. The capacity available for data is half the total of the two disks. Striping alone with no redundancy is RAID 0, and writing parity distributed across disks is RAID 5. Note that RAID protects against disk failures; data deleted by mistake or encrypted by ransomware is reflected on both disks, so RAID is no substitute for backups.

RAID 5 is built from four disks with a capacity of 4 TB each. Which is the capacity available for storing data?

  1. A8 TB
  2. B12 TB
  3. C16 TB
Show answer and explanation

Answer: B. 12 TB

RAID 5 writes data and parity distributed across all the disks and uses one disk's worth of capacity for parity. The capacity available for data is therefore (4 − 1) × 4 TB = 12 TB, and if one disk fails, the lost data can be restored from the data and parity on the remaining disks. 8 TB is the capacity for RAID 1 or RAID 10, which halve the capacity, and 16 TB is the capacity for RAID 0, which has no redundancy.

FAQ

Should I choose RAID 5 or RAID 6?

RAID 6 tolerates two disk failures, so it suits arrays with many disks or large disks whose rebuilds take a long time. In exchange, it loses two disks' worth of capacity and its write performance is lower than RAID 5's. Choose by considering how important the use is, the capacity and number of disks, and the amount of writing.

What is the difference between RAID 1 and RAID 10?

RAID 1 is mirroring, in which two disks hold the same data. RAID 10 creates several such mirrored pairs and splits data across the pairs (striping), increasing capacity and speed. Both can store half of the total capacity.

RAID in the explanations

Questions that mention RAID (5)

  1. Which describes RAID 1?
  2. RAID 5 is built from four disks with a capacity of 4 TB each. Which is the capacity available for storing data?
  3. RAID 6 is built from six disks with a capacity of 8 TB each. Which is the correct combination of the capacity available for storing data and the number of disks that can fail at the same time without losing data?
  4. In a RAID 10 array of four disks (two mirrored pairs, striped across the two pairs), two of the four disks fail at the same time. If every pair of disks is equally likely to be the one that fails, which is the probability that data is lost?
  5. RAID 5 is built from four disks, each capable of 200 random input/output operations per second (IOPS). Each small random write requires four I/O operations in total: reading the old data and the old parity, and writing the new data and the new parity; cache effects are ignored. When only writes are performed, which is the number of writes the whole RAID 5 array can process?

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Semiconductor, Electronics and Computing Calculators

RAID Capacity CalculatorCalculate usable capacity, capacity efficiency, how many disk failures can be tolerated and the chance of data loss for RAID 0, 1, 5, 6 and 10 from the number and size of disks

Related terms

Ransomware Differential backup Dual system Duplex system Fail-safe Fail-soft Fault tolerance Fool-proof Hot standby Incremental backup

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