# Wednesday, 02 September 2015

One of the nice things about Azure storage is that Azure always makes extra copies of your data. How and where those copies are made is up to you.

In the current Azure portal, you select the REPLICATION property of a new Storage Account; In the Azure Preview Portal, you select the Storage Account Type.

In both cases, the options are:

  • Locally Redundant
  • Geo-Redundant
  • Read-Access Geo-Redundant
  • Zone Redundant

Here is an explanation of each type:

Locally Redundant

Three copies of your storage data are created - all within the same region. No two copies will reside in the same Fault Domain and no two copies will reside in the same Upgrade domain.

This provides fault tolerance in case of the failure of one of the machines on which a copy of the storage account is stored. If the entire data center goes down, no copies of your data will be available.

This is the cheapest of the available redundancy options.

Geo-Redundant

As with Locally Redundant storage, Geo-Redundant storage also creates 3 copies of your data on separate fault domains and update domains in the same data center. But it also creates 3 more copies of your data in another region - typically the region nearest the primary region for this account. For example, if you select North Central US as your storage account's primary region, the account data will be replicated in the South Central US Region.

Once this cross-region replication occurs, you are protected from data loss, even if an entire Azure region fails.

Read-Access Geo-Redundant

Read-Access Geo-Redundant storage is identical to Geo-Redundant storage, but it also provides read access to data stored in the secondary region,

Zone Redundant

Three copies of your storage data are created and stored in at least 2 geographically disparate data centers. These data centers may or may not be in the same Region. This provides fault tolerance, even if an entire data center fails.

Zone Redundant Storage Accounts only support Block Blog storage, so selecting this option will limit the uses of your Storage account.

Considerations

Data within a region or data center is always distributed across multiple update domains and fault domains to protect against most hardware failures or planned maintenance downtime.

Replication within a data center is an atomic operation. In other words, success is not reported to the client until all 3 copies have been successfully written.

Replication to a secondary data center is done asynchronously and typically completes after success has been reported to the client. The good news about this is that clients don't experience any latency when writing to one of a Geo-Redundant storage account. A potential downside is that there is that data in the secondary data center is eventually consistent. If the primary data center fails, it is possible that not all data was written yet to the secondary data center.

Geo-Redundant and Read-Access Geo-Redundant are very similar - both create 6 copies of your data spread across two regions. The difference is that in a Geo-Redundant scenario, the data in the secondary region is only accessible in the event of a failure in the primary region. If all 3 copies of the data in the primary region are unavailable, Azure will fail over to a copy of the data in the secondary region. This also holds true with Read-Access Geo-Redundant, but you get one more benefit: users can access a read-only copy of the data in the secondary region, even if there is no failure in the first region. This can make for greater availability and access speed for users. This also explains why Read-Access Geo-Redundant is the most expensive option. It's the only option that allows users to read copies of the data.

Which Should I choose?

For maximum performance and reliability, Read-Access Geo-Redundant storage is your best option. But this is also the most expensive option. If you are very cost-conscious or if the government requires you to keep data within specific geographic boundaries, you should consider Zone Replication. Geo-Redundant storage is a good compromise between these two options for most scenarios.