Storage tiers and redundancy¶
Azure Storage pricing and durability depend on how often data is accessed and how many copies are kept. These two decisions appear constantly on AZ-900.
Access tiers for blob storage¶
| Tier | Access pattern | Cost pattern |
|---|---|---|
| Hot | Frequent reads and writes | Higher storage cost, lower access cost |
| Cool | Infrequent access for at least 30 days | Lower storage cost, higher access cost |
| Cold | Infrequent access for at least 90 days | Lower storage cost than cool, higher retrieval cost |
| Archive | Rare access for at least 180 days; rehydration required | Lowest storage cost, highest retrieval latency and cost |
Exam clue: rarely accessed backups or compliance archives → Cool, Cold, or Archive, depending on how rarely the data is read and how quickly it must be available again.
Archive is offline-style storage. Plan time for rehydration before the data can be read again.
Redundancy options¶
Redundancy determines how Azure copies data inside and across facilities.
| Option | Copies | Protects against |
|---|---|---|
| LRS (Locally redundant storage) | 3 copies in one physical location in the primary region | Hardware failure within that location |
| ZRS (Zone-redundant storage) | 3 copies across availability zones in a region | Zone failure within a region |
| GRS (Geo-redundant storage) | 6 copies: 3 in the primary region and 3 in a geographically distant secondary region | Region-wide disaster; secondary access requires failover unless read access is enabled |
| GZRS | ZRS in primary plus replication to secondary region | Zone failure and regional disaster recovery design |
| RA-GRS / RA-GZRS | Geo-redundant with read access to secondary | Read from secondary region before a full failover in supported scenarios |
Fast rules¶
- Lowest cost, single datacenter acceptable → LRS.
- Need zone resilience for storage in one region → ZRS.
- Need secondary region copy for disaster recovery → GRS or GZRS.
- Need to read from secondary before failover → RA-GRS or RA-GZRS.
Redundancy does not replace an application disaster-recovery design. It protects stored data copies according to the selected option.
When a primary region has an Azure region pair, geo-redundant storage commonly uses that paired region. Because newer Azure regions can be nonpaired, the reliable exam-level description is a geographically distant secondary region, not always "the paired region."
Scenario¶
Regulatory backups must survive a full regional outage and remain readable from the secondary region during an investigation.
Best fit: RA-GRS or RA-GZRS, depending on whether zone redundancy is also required in the primary region.
Scenario¶
Development test files can be recreated easily and cost must stay minimal.
Best fit: LRS with Cool or Hot tier based on access frequency.
Exam clues¶
- Cheapest redundancy → LRS.
- Zone failure inside one region → ZRS or GZRS.
- Secondary region for disaster recovery → GRS, GZRS, RA-GRS, or RA-GZRS.
- Rarely accessed long-term backup → Archive tier.
Check yourself¶
Statement: Archive tier blobs can be read instantly without rehydration.
Answer: False. Archive data must be rehydrated to hot or cool before normal read access.