Why redundancy matters
Disks fail, racks lose power and whole data centres can have a bad day. Azure Storage always keeps several copies of your data. The redundancy setting decides where those copies are kept, and how much failure your data can survive.
The four options
- Locally redundant storage (LRS) keeps three copies in a single data centre in one region. It is the cheapest option and protects against disk and rack failures. It does not protect against the loss of that data centre.
- Zone-redundant storage (ZRS) keeps three copies across three availability zones in one region. Your data stays available if one zone is lost.
- Geo-redundant storage (GRS) keeps three copies in the primary region, as with LRS, and copies the data in the background to a second, paired region, where it is stored three more times. It protects against the loss of a whole region.
- Geo-zone-redundant storage (GZRS) combines ZRS in the primary region with a copy to the paired region. It gives the most protection.
For GRS and GZRS you can also choose read access (RA-GRS and RA-GZRS). The data in the second region can then be read at any time, not only after a failover.
Durability and availability are different
Durability is the chance that your data survives. Microsoft documents it as at least 11 nines for LRS, 12 nines for ZRS and 16 nines for the geo options, measured over a year. Availability is the chance that you can reach the data at a given moment. A higher redundancy level usually improves both, but they are separate numbers, so read the service agreement for the figures that apply to your account.
Which one to choose
- LRS for data you can recreate, test data, or when another copy already exists elsewhere.
- ZRS for production workloads that must keep running through the loss of a data centre in a region that has availability zones.
- GRS or GZRS for data that must survive the loss of a region, such as backups and records with legal retention.
Geo-redundant copies are copied asynchronously, so a regional failover can lose the most recent writes. Plan for that in your recovery targets.
Changing your mind later
You can change an account's redundancy after it is created. Some changes are a setting you update, and some need a conversion request that takes time. Check the current Microsoft documentation before planning a change, and test on a non-production account first.
Today's task
For each storage account from yesterday's task, write down whether its redundancy matches the importance of the data in it.