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DMCP: Storage and Data Protection

DMCP · Work through your exam, one topic at a time.

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Storage and Data Protection

The Storage and Data Protection domain tests whether you can size usable capacity, choose a protection scheme, and place recovery points correctly. Items usually arrive as a scenario - a drive count, a RAID level, a stated RPO/RTO - where the wrong options differ from the right one by a single arithmetic or conceptual step.

What each protection layer actually protects

  • RAID protects against drive failure only. It does not protect against controller loss, deletion, ransomware, or site loss, so it is never a backup.
  • RAID 5 tolerates one drive failure, RAID 6 tolerates two, RAID 10 tolerates one per mirror and rebuilds quickly because it copies from a surviving mirror.
  • Wide parity stripes on 16 TB-and-larger nearline drives mean long rebuilds; a second failure or an unreadable sector during rebuild is a genuine design risk. Mitigate with RAID 6, a hot spare, and scheduled scrubs.

Capacity arithmetic

Usable = raw x (data drives / total drives). Remove the hot spare from the drive count first, then apply snapshot reserve, then apply the utilisation ceiling (commonly 80%) for performance and rebuild headroom. Decimal TB and binary TiB differ by about 9%; if an option is roughly 10% off, check the unit.

Recovery objectives

RPO is how much data you can lose and sets copy frequency. RTO is how long until service returns and sets replication and automation, not backup. Snapshots give a fast RPO and a fast single-file restore but sit on the same platform; replication gives a second copy; only an independent, immutable copy survives ransomware or loss of the array.

Worked example

12 x 16 TB drives, RAID 6, one hot spare, 10% snapshot reserve, 80% utilisation ceiling. Raw = 192 TB. Remove two parity drives and one spare: 9 x 16 = 144 TB. Snapshot reserve: 144 x 0.90 = 129.6 TB. Utilisation cap: 129.6 x 0.80 = 103.7 TB of data. The reasoning that earns the mark is subtracting drives (not a parity percentage) and applying the two reserves multiplicatively in the stated order.

Traps that cost marks

  • Quoting raw capacity as usable, or leaving the hot spare inside the data drive count.
  • Swapping RPO and RTO in a replication scenario.
  • Treating a snapshot as a backup, or treating thin provisioning as added capacity.
  • Declaring large-drive RAID 6 safe without acknowledging rebuild exposure.