The question asks for a type of backup to meet availability requirements. In Azure SQL, the automated backup service that enables Point-in-Time Restore (PITR) uses a combination of full, differential, and transaction log backups. Differential backups are critical for meeting availability requirements, specifically for reducing the Recovery Time Objective (RTO). A differential backup contains only the data that has changed since the last full backup. Using a recent differential backup during a restore process significantly reduces the number of transaction log backups that need to be applied, which makes the overall restore operation faster and brings the database online more quickly.

A. transaction log: While essential for minimizing data loss (low RPO) and enabling point-in-time recovery, relying only on full and transaction log backups can lead to longer restore times.

B. point-in-time restore (PITR): This is a restore capability or method that uses the underlying full, differential, and transaction log backups. It is not a type of backup itself.

D. long-term retention (LTR): LTR is a feature for storing backups for extended periods (up to 10 years) for compliance and archival, not for meeting operational availability and rapid recovery needs.

1. Microsoft Learn

"Automated backups in Azure SQL Database": This document outlines the backup strategy used by Azure SQL. It states

"Azure SQL Database and SQL Managed Instance use SQL Server technology to create full backups every week

differential backups every 12-24 hours

and transaction log backups approximately every 10 minutes." This confirms that differential backups are a fundamental type used in the automated solution that provides high availability.

2. Microsoft Learn

"Differential Backups (SQL Server)": This document explains the purpose of differential backups. Under the "Overview of differential backups" section

it states

"This can speed up the process of restoring a database because fewer log backups have to be applied." This directly supports the reasoning that differential backups improve RTO

a key component of availability.

3. Microsoft Learn

"Recover a database using automated database backups": This page describes the restore process. It confirms that PITR is the method of recovery: "Point-in-time restore (PITR) allows you to restore a database... to an earlier point in time." This distinguishes the restore method from the backup types.

4. Microsoft Learn

"Long-term retention overview": This document clarifies the purpose of LTR. The overview section states

"Many applications have regulatory

compliance

or other business purposes that require you to retain database backups beyond the 1-35 days provided by short-term retention." This positions LTR for archival

not for immediate availability.