In enterprise storage, how does a "Copy-on-Write" (CoW) snapshot differ from a traditional backup?

Question

In enterprise storage, how does a "Copy-on-Write" (CoW) snapshot differ from a traditional backup?

1. A. It is stored on a physically separate secondary device.
2. B. It only stores pointers to data blocks until those blocks are modified. (Correct answer)
3. C. It requires the system to be offline during the process.
4. D. It creates a full bit-by-bit clone of the data immediately.

Correct Answer

Option B — It only stores pointers to data blocks until those blocks are modified.

Detailed Solution & Explanation

The correct answer is It only stores pointers to data blocks and records changes.

Key Points

• A Copy-on-Write snapshot captures the state of a volume by recording pointers to existing blocks rather than duplicating them.
• Original data is copied only when a block is about to be modified — hence "copy on write" — so a snapshot is near-instant and initially consumes almost no space.
• A traditional backup copies the actual data to separate media, taking far longer and occupying full storage.
• Snapshots also need no downtime, which is why the "system must be offline" option is wrong.

Additional Information

• A snapshot is not a substitute for a backup: it depends on the original volume, so losing that storage loses the snapshots with it. Backups are independent copies, ideally held off-site.
• The 3-2-1 rule is the standard guidance — three copies of data, on two different media, with one off-site.
• The alternative technique is Redirect-on-Write, which writes new data elsewhere and leaves the original untouched, generally with lower write overhead.
• Backup types worth distinguishing: full, incremental (changes since the last backup of any kind) and differential (changes since the last full backup).