Redundancy options for managed disks

Applies to: ✔️ Linux VMs ✔️ Windows VMs ✔️ Flexible scale sets ✔️ Uniform scale sets

Azure managed disks offer two storage redundancy options, zone-redundant storage (ZRS), and locally-redundant storage. ZRS provides higher availability for managed disks than locally-redundant storage (LRS) does. However, the write latency for LRS disks is better than ZRS disks because LRS disks synchronously write data to three copies in a single data center.

Locally-redundant storage for managed disks

Locally-redundant storage (LRS) replicates your data three times within a single data center in the selected region. LRS protects your data against server rack and drive failures. To protect an LRS disk from a zonal failure like a natural disaster or other issues, take the following steps:

  • Use applications that can synchronously write data to two zones, and automatically failover to another zone during a disaster.
    • An example would be SQL Server AlwaysOn.
  • Take frequent backups of LRS disks with ZRS snapshots.
  • Enable cross-zone disaster recovery for LRS disks via Azure Site Recovery. However, cross-zone disaster recovery doesn't provide zero Recovery Point Objective (RPO).

If your workflow doesn't support application-level synchronous writes across zones, or your application must meet zero RPO, then ZRS disks would ideal.

Zone-redundant storage for managed disks

Zone-redundant storage (ZRS) synchronously replicates your Azure managed disk across three Azure availability zones in the region you select. Each availability zone is a separate physical location with independent power, cooling, and networking.

A ZRS disk lets you recover from failures in availability zones. If a zone went down, a ZRS disk can be attached to a virtual machine (VM) in a different zone. ZRS disks can also be shared between VMs for improved availability with clustered or distributed applications like SQL FCI, SAP ASCS/SCS, or GFS2. A shared ZRS disk can be attached to primary and secondary VMs in different zones to take advantage of both ZRS and availability zones. If your primary zone fails, you can quickly fail over to the secondary VM using SCSI persistent reservation.

Billing implications

For details see the Azure pricing page.

Comparison with other disk types

Except for more write latency, disks using ZRS are identical to disks using LRS, they have the same scale targets. Benchmark your disks to simulate the workload of your application and compare the latency between LRS and ZRS disks.

Limitations

ZRS for managed disks have the following restrictions:

  • Only supported with premium solid-state drives (SSD) and standard SSDs.
  • Currently available only in the West US 2, West Europe, North Europe, and France Central regions.
  • Can't currently be used with Azure Backup or Azure Site Recovery.

Next steps