Microsoft Azure introduces in the next months a new option for customers to have their data highly available. Zone Redundant Storage is a mix of two currently available options for data redundancy: it replicates data to other datacenters but is limited to three copies.
Currently Microsoft Azure (the cloud service formerly known as Windows Azure) offers customers two choices for keeping data highly available.
1. Locally redundant storage (LRS). Data is stored as three different copies inside the same facility. A facility is a physical datacenter in a single region. Data is replicated synchronously
2. Geo redundant storage (GRS). This is the default option when creating an Azure Storage Account. Data is stored as three different copies inside the same, primary datacenter (like LRS). Additionally, three copies of the data are also stored in a secondary datacenter located in another region. So in total data is stored six times. Replication to the secondary location is performed asynchronously. Distance between the two regions is a couple of hundred miles. In some cases the replicated data is stored in a different county. (Amsterdam-Dublin,Singapore-Hong Kong, Brasil- San Antonio, Texas)
The main purpose for replication is disaster recovery. In case a facility has a technical issue or is unavailable (fire, flooding etc), a copy of the data is available in another region. Mind that this replication is mainly an issurance for Microsoft to meet the SLA. Microsoft staff will decide to failover to the replicated data in case of issues. Customers cannot initiate a failover.
Since the first week of April 2014 Read Access Geo Redundant Storage (RA-GRS) is general available. RA-GRS allows customers to have read access to the data stored in the secondary region. Writes are not possible to this secondary region.
A new, third option for data availability named Zone Redundant Storage (ZRS) will become available in the coming months. It allows data to be replicated to a secondary datacenter/facility located in the same region or to a paired region. Instead of storing 6 copies of data like GRS does, 3 copies of data are stored. So ZRS is a mix of LRS and GRS. Three copies but stored in different datacenters/facilities. I am not sure if all Azure regions have more than one datacenter. If not, the data will replicated to another region.
ZRS will be priced 37.5% lower than GRS as it becomes available.