Keep a VM off Flash

  • 3 May 2019
  • 4 replies

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Is there an ACLI or NCLI command to keep a VM from using flash tier completely?

Best answer by JeremyJ 5 March 2020, 17:00

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I believe the short answer is no. The only way to prevent a system from depending on flash tier storage is by making sure you're not pinning it.

By design any persistent writes and reads will be placed on flash regardless of the configuration. This is just how the flow of data works. An extent written will be moved off of flash tier storage after a period of time when it's considered cold data.

With reads, if a file is constantly being read it will be moved onto SSD to reduce read latency.
You can pose the question to support, but I believe there is no command to bypass flash altogether.
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You can bypass flash per container with an ncli command.
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In terms of Containers the Tier-wise Usage graph displays a pie chart divided into the percentage of container storage space used by each disk tier (SSD and DAS-SATA).



You can get more information from the Storage Containers Summary View document.

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While it’s possible to pin an individual VM to the hot tier using Flash Mode there isn’t currently an option to skip the hot tier or pin a VM to cold tier, at least at the VM level. 

Like was mentioned by @MMSW_DE it is possible to configure this at the container level from NCLI (i.e. for all VMs and files in that container) but this is generally recommended against so the option won’t show up in tab complete from the interactive NCLI.

Because of the potential for negative impact (unexpected severe performance issues) I’d recommend to discuss the use case with your Nutanix SE, or open a support case for the condition you’re hoping to resolve through this change.