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Azure Storage tiers and availability

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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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