Design data storage solutions
Pick the service from the access pattern
Blob storage for unstructured objects and static content; Azure Files when an application needs an SMB or NFS mount (lift-and-shift); managed disks for VM block storage; Queue for simple messaging; Table for key-value; Data Lake Storage Gen2 (blob with a hierarchical namespace) for analytics engines. Get this first step right and most questions collapse to one answer.
Redundancy and tiering
Know LRS, ZRS, GRS, GZRS and the RA- variants: RA-GRS/RA-GZRS add a readable secondary endpoint. Object-level tiers (hot, cool, cold, archive) beat account-level access tiers because lifecycle management rules can move individual blobs. Cool has a 30-day, cold a 90-day and archive a 180-day minimum before early-deletion charges, and archive rehydration takes hours - archive is for compliance retention, never for data an application reads on demand.
Relational and NoSQL
Azure SQL Database is the managed single-database platform (serverless for intermittent workloads, Hyperscale for very large databases and fast restore, elastic pools for many variable-usage databases). Managed Instance is the choice when the workload needs instance-scoped SQL Server features such as cross-database queries, SQL Agent, and linked servers. SQL on a VM is only for OS-level control. Compare vCore (predictable, hybrid benefit eligible) with DTU (bundled, simpler).
For Cosmos DB, remember the consistency ladder (strong, bounded staleness, session, consistent prefix, eventual); session is the default and strong is incompatible with multi-region writes. Partition key choice decides scalability, and throughput is either provisioned (autoscale) or serverless.
Worked example: "A legacy app writes files to a mapped drive and must keep using a UNC path, with on-premises identity for permissions." Answer: Azure Files with identity-based authentication plus Azure File Sync - not Blob, not a managed disk.
Trap: assuming ZRS is always available or that geo-redundancy is readable without the RA- prefix.