SSC CGL 2025 Tier-II · Paper-I, 19 January 2026 · Computer Knowledge Test · Question 19 of 150

We transcribed this text from the pictures of a candidate's response sheet for this paper; the answer is the key SSC marked with a green tick on that sheet. Solution last reviewed 11 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. This paper reaches us as a candidate's response sheet saved as pictures, so we transcribed the text from those pictures, and a second reader checked every question against them; where the layout carries meaning, we show the picture too. Each answer is the key SSC marked with a green tick on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. UnlockIAS wrote the solutions.

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

(a)It only stores pointers to data blocks until those blocks are modified.
(b)It is stored on a physically separate secondary device.
(c)It requires the system to be offline during the process.
(d)It creates a full bit-by-bit clone of the data immediately.

Answer

Why

Correct — A. A copy-on-write snapshot copies no data when it is taken. It starts as a map pointing at the live blocks, and a block's old contents are copied aside for the snapshot just before that block is overwritten.

Unchanged blocks stay shared with the live volume, so the snapshot is quick to create and starts small → option (a). A traditional backup is a separate copy of the data.

Why the others are wrong

(b)
It is stored on a physically separate secondary device. — A physically separate device describes a backup, not a CoW snapshot. The snapshot reads its unchanged blocks from the live volume, so it cannot outlive that volume, while a backup on other media can.
(c)
It requires the system to be offline during the process. — Snapshots are taken online. Windows' Volume Shadow Copy Service, for one, pauses writes for a few seconds while the snapshot is created and lets backups run without taking applications offline.
(d)
It creates a full bit-by-bit clone of the data immediately. — A full bit-by-bit clone made at once is what Microsoft calls a complete copy, or split mirror. Copy-on-write is defined by not doing that: it copies a block only when the block changes.

Concept

A snapshot is a point-in-time view of a volume. Microsoft's Volume Shadow Copy documentation names three ways to make one: a complete copy (a full clone, such as a split mirror), copy-on-write and redirect-on-write.

In copy-on-write, before a write to the original volume completes, each block about to change is read and saved to a separate diff area. The saved old blocks plus the unchanged live blocks rebuild the volume as it was at the snapshot.

Snapshots and backups work together. Microsoft's VSS FAQ says that when data is copied from a shadow copy to tape or other removable media, the copy on the media is the backup, and the shadow copy can then be deleted.

Key facts

Copy-on-write saves a block to the snapshot area just before that block is overwritten.
Windows' built-in VSS system provider uses copy-on-write.
Microsoft lists three snapshot methods: complete copy, copy-on-write and redirect-on-write.
A copy-on-write snapshot is quick to create because only changed data is copied.

Study next

Redirect-on-write snapshotsFull, incremental and differential backupsVolume Shadow Copy Service (VSS) in Windows

Common traps

Picking 'full bit-by-bit clone' because 'copy' is in the name, when copy-on-write copies nothing until data changes.
Treating a snapshot as a backup, although it depends on the live volume for every unchanged block.

The stem contrasts a snapshot with a backup, and the wrong options borrow properties of backups and full clones. Read the name literally, copy on write, and the pointer option follows.

Related PYQs

No directly related past PYQ was found.

Topic: Computer Basics — all 7 SSC CGL questions on it, every solved shift.