Blob rehydration from the archive tier
While a blob is in the archive access tier, that blob is considered to be offline, and can't be read or modified. In order to read or modify data in an archived blob, you must first rehydrate the blob to an online tier, either the hot or cool tier. There are two options for rehydrating a blob that is stored in the archive tier:
Copy an archived blob to an online tier: You can rehydrate an archived blob by copying it to a new blob in the hot or cool tier with the Copy Blob operation.
Change an archived blob's access tier to an online tier: You can rehydrate an archived blob to the hot or cool tier by changing its tier using the Set Blob Tier operation.
Rehydrating a blob from the archive tier can take several hours to complete. Azure recommends archiving larger blobs for optimal performance when rehydrating. Rehydrating a large number of small blobs might require extra time due to the processing overhead on each blob. A maximum of 10 GiB per storage account may be rehydrated per hour with priority retrieval.
To learn how to rehydrate an archived blob to an online tier, see Rehydrate an archived blob to an online tier.
Rehydration priority
When you rehydrate a blob, you can set the priority for the rehydration operation via the optional x-ms-rehydrate-priority header on a Set Blob Tier or Copy Blob operation. Rehydration priority options include:
- Standard priority: The rehydration request is processed in the order it was received and might take up to 15 hours to complete for objects under 10 GB in size.
- High priority: The rehydration request is prioritized over standard priority requests and might complete in less than one hour for objects under 10 GB in size.
To check the rehydration priority while the rehydration operation is underway, call Get Blob Properties to return the value of the x-ms-rehydrate-priority header. The rehydration priority property returns either Standard or High.
Standard priority is the default rehydration option. A high-priority rehydration is faster, but also costs more than a standard-priority rehydration. A high-priority rehydration may take longer than one hour, depending on blob size and current demand. Azure recommends reserving high-priority rehydration for use in emergency data restoration situations.
While a standard-priority rehydration operation is pending, you can update the rehydration priority setting for a blob to High to rehydrate that blob more quickly. For example, if you're rehydrating a large number of blobs in bulk, you can specify Standard priority for all blobs for the initial operation, then increase the priority to High for any individual blobs that need to be brought online more quickly, up to the limit of 10 GiB per hour.
The rehydration priority setting can't be lowered from High to Standard for a pending operation. Keep in mind that updating the rehydration priority setting may have a billing impact.
To learn how to set and update the rehydration priority setting, see Rehydrate an archived blob to an online tier.
For more information on pricing differences between standard-priority and high-priority rehydration requests, see Pricing for Azure Blob Storage.
Copy an archived blob to an online tier
The first option for moving a blob from the archive tier to an online tier is to copy the archived blob to a new destination blob that is in either the hot, cool, or cold tier. You can use the Copy Blob operation to copy the blob. When you copy an archived blob to a new blob in an online tier, the source blob remains unmodified in the archive tier.
You must copy the archived blob to a new blob with a different name or to a different container. You can't overwrite the source blob by copying to the same blob.
By copying a blob from the archive tier to an online tier, you can avoid the early deletion fee that is assessed if you change the tier of a blob from the archive tier before the required 180-day period elapses. For more information, see Archive access tier.
This option can also make sense if there's a lifecycle management policy in effect for the storage account, and the daysAfterLastTierChangeGreaterThan condition isn't added to each tierToArchive action of the policy. In that case, rehydrating a blob with Set Blob Tier operation can result in a scenario where the lifecycle policy moves the blob back to the archive tier after rehydration because the last modified time is beyond the threshold set for the policy. A copy operation leaves the source blob in the archive tier and creates a new blob with a different name and a new last modified time, so there's no risk that the rehydrated blob will be moved back to the archive tier by the lifecycle policy.
Copying a blob from the archive tier can take hours to complete depending on the rehydration priority selected. Behind the scenes, a blob copy operation reads your archived source blob to create a new online blob in the selected destination tier. The new blob might be visible when you list the blobs in the parent container before the rehydration operation is complete, but its tier will be set to archive. The data isn't available until the read operation from the source blob in the archive tier is complete and the blob's contents have been written to the new destination blob in an online tier. The new blob is an independent copy, so modifying or deleting it doesn't affect the source blob in the archive tier.
To learn how to rehydrate a blob by copying it to an online tier, see Rehydrate a blob with a copy operation.
Important
Do not delete the source blob until the rehydration has completed successfully. If the source blob is deleted, then the destination blob may not finish copying. You can handle the event that is raised when the copy operation completes to know when it is safe to delete the source blob. For more information, see Handle an event on blob rehydration.
Rehydrating an archived blob by copying it to an online destination tier is supported within the same storage account only for service versions prior to 2021-02-12. Beginning with service version 2021-02-12, you can rehydrate an archived blob by copying it to a different storage account, as long as the destination account is in the same region as the source account. Rehydration across storage accounts enables you to segregate your production data from your backup data, by maintaining them in separate accounts. Isolating archived data in a separate account can also help to mitigate costs from unintentional rehydration.
The target blob for the copy operation must be in an online tier (hot or cool). You can't copy an archived blob to a destination blob that is also in the archive tier.
The following table shows the behavior of a blob copy operation, depending on the tiers of the source and destination blob.
| Hot tier source | Cool tier source | Archive tier source | |
|---|---|---|---|
| Hot tier destination | Supported | Supported | Supported across accounts in the same region with version 2021-02-12 and later. Supported within the same storage account only for earlier versions. Requires blob rehydration. |
| Cool tier destination | Supported | Supported | Supported across accounts in the same region with version 2021-02-12 and later. Supported within the same storage account only for earlier versions. Requires blob rehydration. |
| Archive tier destination | Supported | Supported | Not supported |
Rehydrate from a secondary region
If you configured your storage account to use read-access geo-redundant storage (RA-GRS), then you can use the Copy Blob operation to rehydrate blobs in the secondary region to another storage account that is located in that same secondary region. See Rehydrate from a secondary region.
To learn more about obtaining read access to secondary regions, see Read access to data in the secondary region.
Change a blob's access tier to an online tier
The second option for rehydrating a blob from the archive tier to an online tier is to change the blob's tier by calling Set Blob Tier. With this operation, you can change the tier of the archived blob to either hot or cool.
Once a Set Blob Tier request is initiated, it can't be canceled. During the rehydration operation, the blob's access tier setting continues to show as archived until the rehydration process is complete. When the rehydration operation is complete, the blob's access tier property updates to reflect the new tier.
To learn how to rehydrate a blob by changing its tier to an online tier, see Rehydrate a blob by changing its tier.
Caution
Changing a blob's tier doesn't affect its last modified time. If there is a lifecycle management policy in effect for the storage account, then rehydrating a blob with Set Blob Tier can result in a scenario where the lifecycle policy moves the blob back to the archive tier after rehydration because the last modified time is beyond the threshold set for the policy.
To avoid this scenario, add the daysAfterLastTierChangeGreaterThan condition to the tierToArchive action of the policy. Alternatively, you can rehydrate the archived blob by copying it instead, as described in the Copy an archived blob to an online tier section. Performing a copy operation creates a new instance of the blob with an updated last modified time, so it won't trigger the lifecycle management policy.
Check the status of a blob rehydration operation
During the blob rehydration operation, you can call the Get Blob Properties operation to check its status. To learn how to check the status of a rehydration operation, see Check the status of a rehydration operation.
Handle an event on blob rehydration
Rehydration of an archived blob might take up to 15 hours, and it's inefficient to repeatedly poll Get Blob Properties to determine whether rehydration is complete. Azure recommends that you use Azure Event Grid to capture the event that fires when rehydration is complete for better performance and cost optimization.
Azure Event Grid raises Microsoft.Storage.BlobTierChanged event on the completion of blob rehydration:
- The Microsoft.Storage.BlobTierChanged event fires when a blob's tier is changed. In the context of blob rehydration, this event fires when the access tier of a destination blob is successfully changed from archive tier to an online tier (hot, cool, or cold tier). You can use Set Blob Tier operation to change the access tier of an archived blob or use Copy Blob operation to copy an archived blob to a new destination blob in an online tier.
To learn how to capture an event on rehydration and send it to an Azure Function event handler, see Run an Azure Function in response to a blob rehydration event.
For more information on handling events in Blob Storage, see Reacting to Azure Blob storage events and Azure Blob Storage as Event Grid source.
Pricing and billing
A rehydration operation with Set Blob Tier is billed for data read transactions and data retrieval size. A high-priority rehydration has higher operation and data retrieval costs compared to standard priority. High-priority rehydration shows up as a separate line item on your bill. If a high-priority request to return an archived blob that is less than 10 GB in size takes more than five hours, you won't be charged the high-priority retrieval rate. However, standard retrieval rates still apply.
Copying an archived blob to an online tier with Copy Blob is billed for data read transactions and data retrieval size. Creating the destination blob in an online tier is billed for data write transactions. Early deletion fees don't apply when you copy to an online blob because the source blob remains unmodified in the archive tier. High-priority retrieval charges do apply if selected.
Blobs in the archive tier should be stored for a minimum of 180 days. Deleting or changing the tier of an archived blob before the 180-day period elapses incurs an early deletion fee. For example, if a blob is moved to the archive tier and then deleted or moved to the hot tier after 45 days, you'll be charged an early deletion fee equivalent to 135 (180 minus 45) days of storing that blob in the archive tier. For more information, see Archive access tier.
For more information about pricing for block blobs and data rehydration, see Azure Storage Pricing. For more information on outbound data transfer charges, see Data Transfers Pricing Details.