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CompTIA A+ Core 1

RAID 0, 1, 5, 6 and 10 explained for A+ Core 1

RAID 0 stripes data across drives with no redundancy, so any drive failure loses data. RAID 1 mirrors every block onto more than one drive. RAID 5 survives one drive failure using parity, RAID 6 survives any two, and RAID 10 stripes across mirrored sets and survives one failure in each mirrored set.

Striping, mirroring and parity

Striping spreads consecutive blocks of data evenly across a set of drives. SNIA defines RAID 0 this way and notes it offers no redundancy at all.

Mirroring stores each block on more than one independent drive. SNIA defines RAID 1 this way and says it survives a single drive failure without data loss.

Parity stores extra calculated blocks alongside the data so a lost drive's contents can be worked out from the rest. RAID 5 and RAID 6 are the parity levels.

Each level and what it survives

RAID 0: data striped across all drives, no copies, no parity. Any single drive failure loses data. Use it only for data you can recreate or have backed up, such as a scratch disk.

RAID 1: every block mirrored on more than one drive. Survives a single drive failure.

RAID 5: each stripe holds n data blocks and one parity block across n+1 drives, with parity and data interleaved across all of them. Survives a single drive failure.

RAID 6: parity-based, and survives any two drive failures.

RAID 10, also written RAID 1/0: a stripe (RAID 0) built over a set of mirrors (RAID 1). Survives a single drive failure in each mirrored set. Two failures in the same mirrored set lose data.

Picking a level from a scenario

Read the scenario for two facts: whether parity is mentioned, and how many failures the array must survive. Parity plus one failure is RAID 5. Parity plus any two failures is RAID 6.

"Stripe across mirrored sets" is RAID 10. "Identical copy on more than one drive" with no striping is RAID 1. "Spread evenly, no copies or parity" is RAID 0.

Where the exam catches people

  • Believing RAID 0 adds protection: it has no redundancy, and the failure of any one drive loses data.
  • Believing RAID 5 keeps parity on one dedicated drive: SNIA's definition interleaves parity and data across all the drives.
  • Believing RAID 5 survives two failures: it survives one; RAID 6 is the parity level that survives any two.
  • Believing RAID 10 survives any two failures: it survives one failure in each mirrored set, so losing both drives of one mirror loses data.

Written from CompTIA's documentation

Try 3 questions on this

Pick an answer to see why each option is right or wrong.

  1. A technician sets up a server array in which each stripe holds data blocks plus one parity block, with the parity spread across all of the drives. The array must keep every file if any single drive fails. Which of the following RAID levels is this?

    Right. In the game that’s Renown for your Hardware building.

    Not that one: C is right. In the game your Hardware building would lose a little Renown, and this question would come back later.

    Written from www.snia.org/education/online-dictionary/term/raid-5

  2. A technician needs a disk array that stripes data across sets of mirrored drives, so it keeps all data even if one drive in each mirrored set fails. Which of the following RAID levels should the technician configure?

    Right. In the game that’s Renown for your Hardware building.

    Not that one: B is right. In the game your Hardware building would lose a little Renown, and this question would come back later.

    Written from www.snia.org/education/online-dictionary/term/raid-10

  3. A technician needs a parity-based disk array whose stored data survives any two drive failures without data loss. Which of the following RAID levels should the technician choose?

    Right. In the game that’s Renown for your Hardware building.

    Not that one: C is right. In the game your Hardware building would lose a little Renown, and this question would come back later.

    Written from www.snia.org/education/online-dictionary/term/raid-6