Azure Storage tiers and availability

In this post, we will cover what are the main differences between all the storage tiers available inside Azure Storage.
With the launch of Geo Zone Redundant Storage, there are 6 different types of tiers, that can cover different scenarios. Let's take on different scenarios and see which of them are most suitable for different cases.

Whare are the 6 different tiers?

Locally redundant storage (LRS)
Geo-redundant storage (GRS)
Read Access geo-redundant storage (RAGRS)
Zone-redundent storage (ZRS)
Geo Zone Redundant Storage (GZRS)
Read Access Geo Zone Redundant Storage (RAGZRS)

Remember that all of them are having an availability SLA to write access that has 3 nines (99.9%). The read SLA is different for each tier and starts from 99.9% for LRS, GRS, ZRS and GZRS and goes up to 99.99% for RAGRS and RAGZRS. The durability SLA for a year is at least 11 nines for LRS and goes up to 16 nines for GRS, RAGRS, GZRS and RAGZRS.

Scenario 1: One storage node is not available inside the Availability Zone

For all 6 storage tiers, this case is covered because there at least 3 replicas of the storage on different nodes. The unavailability of one storage node is not affecting access to the content.

Scenario 2: An Availability Zone is down

Locally redundant storage (LRS)

High impact, content is not available anymore as long as the Availability Zone is down

Geo-redundant storage (GRS) & Read Access geo-redundant storage (RAGRS)

Content is still available.
Even so, a failover needs to be triggered to make content available in the secondary locations, that would become the primary one (DNS entries need to be updated)
To identify the real value behind the RPO the Last Sync Time property can be used.
Using Last Sync Time we can identify what data was lost

Zone-redundent storage (ZRS) & Geo Zone Redundant Storage (GZRS) & Read Access Geo Zone Redundant Storage (RAGZRS)

No impact, content is available and ready to be consumed
Content is available in different Availability Zones

Scenario 3: An entire region is not available

Locally redundant storage (LRS)

High impact, content is not available anymore.
There are no replicas in other regions

Geo-redundant storage (GRS) & Read Access geo-redundant storage (RAGRS)

Content is still available
Failover needs to be triggered to make content available from the secondary region
Last Sync Time property can be used to identify what data was lost

Zone-redundent storage (ZRS)

High impact, content is not available. All 3 replicas were done on different Availability Zones from the same region

Geo Zone Redundant Storage (GZRS) & Read Access Geo Zone Redundant Storage (RAGZRS)

In both cases the replicas in another region are available, but the manual trigger for the failover procedure needs to be done
For RAGZRS the secondary node already supports reads operations, but failover is required to support write operations

What tier should I use?

Deciding what type of tier to use is hard and it is all the time a tradeoff between costs and features. Beside this tiers, we have the access tiers (hot, cool and archive). Hard choice.

In the next post, we will take some real-life scenarios and we will indentify the most suitable tier.

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