Geo-redundant storage (GRS)

Geo-redundant storage (GRS) copies your data synchronously three times within a single physical location in the primary region using LRS. It then copies your data asynchronously to a single physical location in a secondary region that is hundreds of miles away from the primary region. GRS offers durability for Azure Storage data objects of at least 99.99999999999999% (16 9's) over a given year.

A write operation is first committed to the primary location and replicated using LRS. The update is then replicated asynchronously to the secondary region. When data is written to the secondary location, it's also replicated within that location using LRS.

Important

Because data is replicated to the secondary region asynchronously, a failure that affects the primary region may result in data loss if the primary region cannot be recovered. The interval between the most recent writes to the primary region and the last write to the secondary region is known as the recovery point objective (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 it takes to replicate data to the secondary region.

Check the Last Sync Time property

Because data is replicated to the secondary region asynchronously, the secondary region is often behind the primary region. If a failure happens in the primary region, it's likely that all writes to the primary will not yet have been replicated to the secondary.

To determine which write operations have been replicated to the secondary region, your application can check the Last Sync Time property for your storage account. All write operations written to the primary region prior to the last sync time have been successfully replicated to the secondary region, meaning that they are available to be read from the secondary. Any write operations written to the primary region after the last sync time may or may not have been replicated to the secondary region, meaning that they may not be available for read operations.

You can query the value of the Last Sync Time property using Azure PowerShell, Azure CLI, or one of the Azure Storage client libraries. The Last Sync Time property is a GMT date/time value. For more information, see Check the Last Sync Time property for a storage account.

SOURCE : https://docs.microsoft.com/en-us/azure/storage/common/storage-redundancy