Geo-redundant storage (GRS): Cross-regional replication for Azure Storage
Geo-redundant storage (GRS) is designed to provide at least 99.99999999999999% (16 9's) durability of objects over a given year by replicating your data to a secondary region that is hundreds of miles away from the primary region. If your storage account has GRS enabled, then your data is durable even in the case of a complete regional outage or a disaster in which the primary region isn't recoverable.
If you opt for GRS, you have two related options to choose from:
- GRS replicates your data to another data center in a secondary region, but that data is available to be read only if Microsoft initiates a failover from the primary to secondary region.
- Read-access geo-redundant storage (RA-GRS) is based on GRS. RA-GRS replicates your data to another data center in a secondary region, and also provides you with the option to read from the secondary region. With RA-GRS, you can read from the secondary region regardless of whether Microsoft initiates a failover from the primary to secondary region.
For a storage account with GRS or RA-GRS enabled, all data is first replicated with locally redundant storage (LRS). An update is first committed to the primary location and replicated using LRS. The update is then replicated asynchronously to the secondary region using GRS. When data is written to the secondary location, it's also replicated within that location using LRS.
Both the primary and secondary regions manage replicas across separate fault domains and upgrade domains within a storage scale unit. The storage scale unit is the basic replication unit within the datacenter. Replication at this level is provided by LRS; for more information, see Locally redundant storage (LRS): Low-cost data redundancy for Azure Storage.
Keep these points in mind when deciding which replication option to use:
- Zone-redundant storage (ZRS) provides highly availability with synchronous replication and may be a better choice for some scenarios than GRS or RA-GRS. For more information on ZRS, see ZRS.
- Asynchronous replication involves a delay from the time that data is written to the primary region, to when it is replicated to the secondary region. In the event of a regional disaster, changes that haven't yet been replicated to the secondary region may be lost if that data can't be recovered from the primary region.
- With GRS, the replica isn't available for read or write access unless Microsoft initiates a failover to the secondary region. In the case of a failover, you'll have read and write access to that data after the failover has completed. For more information, please see Disaster recovery guidance.
- If your application needs to read from the secondary region, enable RA-GRS.
Read-access geo-redundant storage
Read-access geo-redundant storage (RA-GRS) maximizes availability for your storage account. RA-GRS provides read-only access to the data in the secondary location, in addition to geo-replication across two regions.
When you enable read-only access to your data in the secondary region, your data is available on a secondary endpoint as well as on the primary endpoint for your storage account. The secondary endpoint is similar to the primary endpoint, but appends the suffix
–secondary to the account name. For example, if your primary endpoint for the Blob service is
myaccount.blob.core.windows.net, then your secondary endpoint is
myaccount-secondary.blob.core.windows.net. The access keys for your storage account are the same for both the primary and secondary endpoints.
Some considerations to keep in mind when using RA-GRS:
- Your application has to manage which endpoint it is interacting with when using RA-GRS.
- Since asynchronous replication involves a delay, changes that haven't yet been replicated to the secondary region may be lost if data can't be recovered from the primary region.
- You can check the Last Sync Time of your storage account. Last Sync Time is a GMT date/time value. All primary writes before the Last Sync Time have been successfully written to the secondary location, meaning that they are available to be read from the secondary location. Primary writes after the Last Sync Time may or may not be available for reads yet. You can query this value using the Azure portal, Azure PowerShell, or from one of the Azure Storage client libraries.
- If you initiate an account failover (preview) of a GRS or RA-GRS account to the secondary region, write access to that account is restored after the failover has completed. For more information, see Disaster recovery and storage account failover (preview).
- RA-GRS is intended for high-availability purposes. For scalability guidance, review the performance checklist.
- For suggestions on how to design for high availability with RA-GRS, see Designing Highly Available Applications using RA-GRS storage.
What is the RPO and RTO with GRS?
Recovery Point Objective (RPO): In GRS and RA-GRS, the storage service asynchronously geo-replicates the data from the primary to the secondary location. In the event that the primary region becomes unavailable, you can perform an account failover (preview) to the secondary region. When you initiate a failover, recent changes that haven't yet been geo-replicated may be lost. The number of minutes of potential data that's lost is known as the RPO. The RPO indicates the point in time to which data can be recovered. Azure Storage typically has an RPO of less than 15 minutes, although there's currently no SLA on how long geo-replication takes.
Recovery Time Objective (RTO): The RTO is a measure of how long it takes to perform the failover and get the storage account back online. The time to perform the failover includes the following actions:
- The time until the customer initiates the failover of the storage account from the primary to the secondary region.
- The time required by Azure to perform the failover by changing the primary DNS entries to point to the secondary location.
When you create a storage account, you select the primary region for the account. The paired secondary region is determined based on the primary region, and can't be changed. For up-to-date information about regions supported by Azure, see Business continuity and disaster recovery (BCDR): Azure paired regions.
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