[6.9.1] Questions about cache pool operation and 'the mover' as they relate to VM's


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Hey all,

I am fairly new to Unraid, and while this seems like a great time to have started using it with the 6.9x release, I seem to be having trouble finding information on how some things work under the hood now.

 

I was running a 970 Evo Plus NVME as my cache drive, but today I grabbed another one. I was going to put them in raid 0, but now I am not sure as I saw some mention about 'the mover'. I am really not sure exactly how the mover works, or what criteria is used as the basis of it's operation, or how to influence that. That said, does having the cache in raid 0 cause additional restriction on how/when the mover is able to operate vs using raid 1 or will that not make a difference? Are there any functionality differences between using cache in raid 0 vs 1 (outside of the basics of standard raid operation) when it comes to what unraid can and cannot do under the hood while in either mode?

 

Since I just rebuilt the cache (added the second 970 about an hour ago) I started the VM back up but it felt magnificently unusable, so do I have to manually invoke the mover to have the vm run on the cache pool? If so, how does one let it know when running it, that the VM is what I want to speed up?

 

Lastly, when a VM is using the cache pool, does it then only live in the cache pool, or are changes written back to the array? If so, how often does that happen? Does it only happen when the VM is off and that is what the mover actually does? So many questions on how things work under the hood, but can't seem to really find much current info.

 

I appreciate any clarification that anyone might be able to provide.

Thanks,

-MH

Edited by MostHated
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  • MostHated changed the title to [6.9.1] Questions about cache pool operation and 'the mover' as they relate to VM's

The mover is a Unraid function that moves files from a cache pool to the array or from the array to the cache. It's used mainly in the situation where downloads or file copies are placed on the cache pool by default but then are moved to the array when the cache fills up. Mover operations are defined at the share level. If you go to the shares tab you'll see all your shares (folders at the root of the array or pools/cache drives). If you click into a share, you can configure the "Use cache pool" option. The help text is as follows for the 4 options:

 

Quote

No prohibits new files and subdirectories from being written onto the Cache disk/pool. Mover will take no action so any existing files for this share that are on the cache are left there.

 

Yes indicates that all new files and subdirectories should be written to the Cache disk/pool, provided enough free space exists on the Cache disk/pool. If there is insufficient space on the Cache disk/pool, then new files and directories are created on the array. When the mover is invoked, files and subdirectories are transferred off the Cache disk/pool and onto the array.

 

Only indicates that all new files and subdirectories must be writen to the Cache disk/pool. If there is insufficient free space on the Cache disk/pool, create operations will fail with out of space status. Mover will take no action so any existing files for this share that are on the array are left there.

 

Prefer indicates that all new files and subdirectories should be written to the Cache disk/pool, provided enough free space exists on the Cache disk/pool. If there is insufficient space on the Cache disk/pool, then new files and directories are created on the array. When the mover is invoked, files and subdirectories are transferred off the array and onto the Cache disk/pool.

 

Most people will probably use the "Yes" option (for using the cache as a temp landing ground) or "Only" option (for times where you want files to always be on fast storage).

 

I'd suggest for your share that hosts the vm files to use the "Only" option. However, at first use the "Prefer" option. Then shutdown your vms. Use the mover (this will copy the files from the array to the cache pool). Turn your VMs back on and then the share to "Only".

 

Note: Files will never live on both the array and cache pool at that same time (unless you copy the files manually, not suggested). So this isn't a back up solution, all this defines is where you want files to live. If you want a backup solution, you'll need to do something different like a plugin that will do VM backups for you (se below). Or using BTRFS snapshots + btrfs send to a backup server.

 

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