Blob Access Tiers (Hot, Cool, Cold, Archive)

Azure Blob Storage offers multiple access tiers that let you optimise storage costs based on how frequently you access your data. By placing data in the right tier, you can significantly reduce your storage bill without sacrificing availability when you need it. This lesson covers each tier, their cost trade-offs, and how to move data between them.

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Why Access Tiers Matter

Not all data is accessed equally. An e-commerce site might frequently read product images but rarely touch old order archives. A compliance system might store audit logs that are read once a year. Azure access tiers let you match storage costs to access patterns.

The principle is simple:

• Frequently accessed data — pay more per GB stored, less per operation
• Rarely accessed data — pay less per GB stored, more per operation

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The Four Access Tiers

Tier	Storage Cost	Access Cost	Min Retention	Availability SLA
Hot	Highest	Lowest	None	99.9% (99.99% RA-GRS)
Cool	Lower	Higher	30 days	99% (99.9% RA-GRS)
Cold	Even lower	Even higher	90 days	99% (99.9% RA-GRS)
Archive	Lowest	Highest	180 days	Offline (not directly accessible)

Hot Tier

The Hot tier is optimised for data that is accessed or modified frequently.

Use cases:

• Active application data
• Media files being served to users
• Data undergoing processing
• Frequently read databases or data sets

Characteristics:

• Highest per-GB storage cost
• Lowest per-transaction cost
• Data is immediately accessible
• Can be set as the default account access tier

Cool Tier

The Cool tier is for data that is infrequently accessed and stored for at least 30 days.

Use cases:

• Short-term backup and disaster recovery
• Data sets that are no longer actively used but need quick access when requested
• Recently ingested data that will be processed later

Characteristics:

• Lower storage cost than Hot (~40-50% cheaper per GB)
• Higher access and transaction costs
• Early deletion penalty if deleted before 30 days
• Data is immediately accessible

Cold Tier

The Cold tier is designed for data that is rarely accessed and stored for at least 90 days.

Use cases:

• Medium-term backups
• Data retained for compliance but rarely needed
• Seasonal data accessed only during specific periods

Characteristics:

• Lower storage cost than Cool
• Higher access costs than Cool
• Early deletion penalty if deleted before 90 days
• Data is immediately accessible
• Available for block blobs in general-purpose v2 accounts

Archive Tier

The Archive tier offers the lowest storage cost but the highest access cost and latency.

Use cases:

• Long-term backup retention
• Regulatory and compliance archives
• Original raw data that must be preserved but is almost never read
• Historical data sets

Characteristics:

• Lowest per-GB storage cost (~90% cheaper than Hot)
• Data is offline — not immediately accessible
• Must be rehydrated to Hot, Cool, or Cold before it can be read
• Rehydration can take up to 15 hours (standard priority) or under 1 hour (high priority, at extra cost)
• Early deletion penalty if deleted before 180 days
• Available for block blobs only

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Setting Access Tiers

Account-Level Default Tier

The default access tier is set at the storage account level and applies to any blob uploaded without an explicit tier:

# Set the default account tier to Cool
az storage account update \
  --name mystorageaccount \
  --resource-group rg-storage-demo \
  --access-tier Cool