Azure File Storage

Azure Storage Replication Explained: LRS, ZRS, GRS, RA-GRS

What is Azure Storage Replication?

To maintain data availability and durability, Azure Storage creates and stores copies of data across multiple locations. This process is called storage replication. The goal is to provide redundancy to protect data against hardware failures, power or network outages.

There are several redundancy options you can choose from. You can replicate data within one region, or you can replicate into a geographically-distant secondary region. Additionally, you can enable read access to replicated data located in a secondary region, to provide availability during disasters.

In this article, you will learn:

Types of Azure Storage Replication

By default, Azure Storage replicates data three times within a primary region. Additionally, Azure offers the following options you can configure for replication within the primary region:

  • Locally redundant storage (LRS)—synchronously replicates data to three disks within a data center in the primary region. Offers a moderate level of availability at a lower cost.
  • Zone-redundant storage (ZRS)—synchronously replicates data amongthree Azure availability zones in the primary region. Provides a higher level of resilience at higher cost.

Beyond these two basic types of replication, there are three additional types available in Azure Storage:

  • Geo-Redundant storage (GRS)—stores another three copies of data in a paired Azure region
  • Read-Access Geo-Redundant (RA-GRS)—same as GRS, but allows data to be read from both Azure regions
  • Object Replication for Block Blob Storage—a special type of replication used only for block blobs, replicating them between a source and target storage account.

Locally-Redundant Storage (LRS)

LRS replicates data three times within one data center located in a primary region. When LRS is enabled, Azure Storage only registers write requests as successful once data is written to three replicas. LRS provides at least 99.999999999% durability for objects during a given year.

LRS is offered at a low cost and can help you protect data against drive failure and server rack failure. However, LRS does not cover all data loss scenarios. Use LRS if it is less sensitive to loss, or if your data is restricted to one location due to compliance requirements.

Zone-Redundant Storage (ZRS)

ZRS performs replication across three Azure availability zones. Each Azure availability zone is an individual physical location with its own independent networking, power, and cooling. ZRS provides a minimum of 99.9999999999% durability for objects during a given year.

ZRS ensures data remains accessible even if an availability zone goes down. This is true both for write and read operations. Azure Storage only registers a write operation as successful, once it makes sure data is safely written to all three availability zones. You can use ZRS in a primary region to ensure consistency, high availability, and durability.

Geo-Redundant Storage (GRS)

GRS provides additional redundancy for data storage compared to LRS or ZRS. In addition to the three copies of data stored in one region, there are three copies stored in a paired Azure region. So GRS provides all the features of LRS storage in the primary zone, and additionally, provides a secondary LRS data storage in another region.

Because all read and write operations are still managed through one Azure data center, the read and write SLAs provided by Microsoft are the same as for ZRS and LRS data storage.

There are two disadvantages of GRS redundancy:

  • Replication between regions is asynchronous and so data is propagated with a small delay
  • The second region cannot be accessed or read until the storage account fails over

Read-Access Geo-Redundant (RA-GRS)

RA-GRS has all the same level of redundancy of standard GRS replication, with an additional benefit—the secondary copies stored in paired Azure regions are readable. This means that if your application is configured correctly, you can use multiple readable endpoints. This increases the SLA for read operations to 99.99%.

However, the SLA for write operations remains 99.9%, because a single area still controls write and update operations.

Due to their asynchronous replication, both types of GRS replication have some replication delay. You can use the LastSyncTime parameter to ensure you are reading the latest copy of the data.

Object Replication for Block Blob Storage

The preceding replication methods were relevant for all Azure storage services. This is a special replication method which is available only for Block Blob Storage. 

The object replication method is asynchronous. You can use it to automatically move data to an archive tier, in order to optimize data distribution and reduce costs. Or, you can use it to synchronize data to a storage resource running nearer to your users, to reduce latency.

Block blobs are replicated according to your replication policy, which specifies source/target Azure accounts and containers, and which block blobs should be replicated.

Block blob object replication copies:

  • Blob content
  • Blob version
  • Blob metadata

Azure Storage Replication Q&A

How to Check Azure Replication Status for Block Blob Storage?

You can check status for block blob replication by looking at the source storage account. If the source account's blob replication status indicates failure, investigate the following:

  • Check that there is an object replication policy on the target storage account.
  • Make sure the target container exists.
  • If the original blob is encrypted during the write operation using a user-provided key, replication will fail.

Which Replication Types are Available for My Storage Account?

Here are the replication types available, depending on your storage account type:

  • General-purpose v1 supports LRS and GRS/RA-GRS
  • General-purpose v2 supports LRS, ZRS, GRS/RA-GRS, and GZRS/RA-GZRS
  • Block blob storage supports only LRS
  • Blob storage supports LRS and GRS/RA-GRS
  • File storage supports LRS and ZRS

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