Host Caching in Azure came from SCSI RAID controllers with RAM on them, when we had physical machines. The idea is that the disk does not have to wait for everything to be read or written onto the disk before continuing, because the RAID controller has it queue in its dedicated physical memory so it can:
- complete a “write” to the physical drive in (typically) less than a second
- keep frequently requested “reads” in the RAID controllers physical RAM, so it does not need to search for them (yes, even faster than normal SSD)
Should Host Caching Be Enabled on Azure VM’s
HOST CACHING in Azure should, generally, be enabled on Virtual Machines:
- Improved Read Performance: Host caching allows frequently accessed data to be stored in the VM’s memory, reducing the time it takes to read data from the disk
- Reduced Latency: By caching data closer to the VM, you can reduce the latency for read operations, which is particularly beneficial for applications that are sensitive to delays
- Optimized Workloads: Host caching is ideal for workloads that frequently access the same data, such as database servers, web servers, and content delivery networks
- Flexible Configuration: You can configure host caching for both OS disks and data disks, with options for read-only or read/write caching depending on your workload requirements
However, it’s important to note that host caching may not be suitable for all workloads. For example, workloads that write frequently to the disk or cannot tolerate any data loss (i.e. if the VM crashes frequently) should avoid using host caching.
Why is HOST CACHING Greyed Out?
As you can see below there are times when HOST CACHING is grayed out:
In this case the reason was that Version 2 or PREMIUM SSD is so fast it does not improve with Host Caching. Here are the other reasons Host Caching might be grayed out:
- Disk Type: Host caching is only available for certain types of disks. If the disk attached to your VM doesn’t support host caching, the option will be greyed out.
- Disk State: If the disk is in use or performing operations, the host caching option might be temporarily unavailable.
- Permissions: You might not have the necessary permissions to change the host caching settings. Ensure you have the appropriate role assignments in Azure.
- VM Size: Some VM sizes do not support host caching for certain types of disks. Check the VM size and its capabilities.
- Configuration: If the disk was attached with specific configurations that do not allow host caching, the option will be greyed out.
Should VM’s Be Powered Down To Enable / Disable Host Caching?
No, you don’t need to power down the VM to enable host caching. You can enable or change the host caching settings while the VM is running. However, it’s always a good practice to ensure that any critical operations are paused or completed before making changes to the VM settings to avoid any potential disruptions.
