← VCE Software Development U3O2

Differential backup

Copies all files changed since the last full backup. It grows each day, and a restore needs the full backup plus only the latest differential.

Part of: VCE Software Development - U3O2 - Analysis and Design — Module 79, VCE Data Analytics - U3O2 - Analysis and Design — Module 116, VCE Data Analytics - U4O2 - Cyber Security: Data Security — Module 84

Understanding Differential backup in VCE Software Development U3O2

What it does: Each differential contains everything changed since the full backup, so its size increases day by day until the next full backup. The Wednesday differential already contains Monday and Tuesday changes, so earlier differentials are not needed.

Common mistakes: A differential backup needs a full backup of all files as its starting point. Line 1 should be 'Sun: full backup of all files'. A differential backup grows each day because it keeps every change since the full backup. Tuesday's backup cannot be smaller than Monday's, so it should be 3 GB.

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This module's key terms are also in a printable crossword (Modules 72–85) and word search (Modules 52–85).

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Learning
Duo
A differential backup copies files changed since which point?
The last full backupThe previous backup of any type
Matches
Restore = Full + Mon + Tue + Wed differentials
↓
Restoring with every differential is wrong. Only the latest differential is needed because it contains all earlier changes.
Applying
Meanings
Term
differential backup
Meaning
Copies all files changed since the last full backup. It grows each day, and a restore needs the full backup plus only the latest differential.
Practice
BEGIN DifferentialBackup
Testing
Gaps
file TO backup
Check Answer
Spots
1Sun: incremental backup only
2Mon: copy changes since Sun

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Quick Questions

1. A differential backup copies files changed since which point?
A. The previous backup of any type
B. The last full backup ✓
A differential always counts changes from the last FULL backup. An incremental counts changes from the previous backup of any type.
2. Full backup on Sunday. A differential is taken Monday, Tuesday and Wednesday. Which differential is the largest?
A. Monday's, because it was taken first
B. Wednesday's, because it holds all changes since Sunday ✓
Each differential repeats earlier changes and adds new ones, so it grows each day until the next full backup.
3. To restore on Thursday after a Sunday full backup and daily differentials, which files do you need?
A. The Sunday full backup plus Wednesday's differential ✓
B. The Sunday full backup plus the Monday, Tuesday and Wednesday differentials
Only the latest differential is needed because it already contains every change since the full backup.
4. True or false: a differential backup grows larger each day until the next full backup.
A. True ✓
B. False
Files changed earlier in the cycle are copied again each day, so the backup keeps growing.
5. Which statement describes a differential backup?
A. It copies all files changed since the last full backup ✓
B. It copies only files changed since the last differential backup
Copying only changes since the previous backup describes an incremental backup, not a differential.

Match the Pairs

differential backup → Copies every file that has changed since the last full backup, not since the last differential
Full + Differential (Fri) → The two backups needed to restore the system when using a differential backup strategy
A differential backup grows each day because it keeps including earlier changes as well as new ones → Full: 10 GB, Mon: 1 GB, Tue: 2 GB, Wed: 3 GB
To restore from differential backups you only need the full backup and the latest differential → restore = Full (Sun) + Differential (Wed)
Restore = Full + Mon + Tue + Wed differentials → Restoring with every differential is wrong. Only the latest differential is needed because it contains all earlier changes.
Tue differential = files changed since Mon differential → A differential backup compares to the last full backup, not the previous differential. That describes an incremental backup.

Key Term

differential backup
Copies all files changed since the last full backup. It grows each day, and a restore needs the full backup plus only the latest differential.

Fill in the Blanks

1. IF file.modified > lastFull THEN COPY file TO backup END IF
Hint: Type the keyword that duplicates a changed file into the backup.
2. IF file.modified > lastFull THEN COPY file TO backup END IF
Hint: Type the variable holding the time of the last full backup, which is what a differential compares against.
3. RESTORE(fullBackup, latestDiff)
Hint: Type the variable for the one differential backup needed with the full backup, the most recent one.
4. diffSize ← diffSize + todaysChanges
Hint: Type the operator that makes the differential backup bigger each day as changes pile up.
5. RESTORE(fullBackup, latestDiff)
Hint: Type the variable for the base backup that every differential restore must start from.

Spot the Error

Question 1
A Sun: incremental backup only
B Mon: copy changes since Sun
C Tue: copy changes since Sun
Line A — A differential backup needs a full backup of all files as its starting point. Line 1 should be 'Sun: full backup of all files'.
Question 2
A Full backup on Sunday
B Mon diff: 2 GB
C Tue diff: 1 GB
Line C — A differential backup grows each day because it keeps every change since the full backup. Tuesday's backup cannot be smaller than Monday's, so it should be 3 GB.
Question 3
A restore = full + every diff
B 1: restore the full backup
C 2: restore the latest diff
Line A — Each differential already contains all changes since the full backup. A restore needs only the full backup plus the latest differential, not every one.
Question 4
A IF day = Sunday THEN
B run DIFFERENTIAL backup
C ELSE
D run DIFFERENTIAL backup
E END IF
Line B — A differential backup must be based on a full backup. The full backup is made on Sunday, so line 2 should be ' run FULL backup'.
Question 5
A Sun: full backup
B Mon-Wed: differentials
C Thu: server crashes
D restore full + Mon diff
Line D — The latest differential (Wednesday's) holds all changes since the full backup. Using Monday's would lose the Tuesday and Wednesday changes.

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