Docker containers accessing disk every 4 seconds when they should be using cache; why?


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Hi everyone,

 

I've been using Unraid for about a month and this is my first post!  Great product and I am enjoying it so far.  The documentation is wonderful and it has been a great experience setting up my own NAS using some spare hardware.  It feels good to re-purpose an old PC!

 

One question about Docker:  Why are my docker containers frequently accessing the physical disks, when they should be using cache?  I have reviewed the docker reference and don't understand why this is happening.  https://wiki.unraid.net/Manual/Docker_Management

 

More details:  I have a 1TB cache pool (two NVME drives) & I have set up the appdata, domains and system shares to "Prefer: Cache" setting.  I understand this to mean that new content for Docker would write to cache, *except* if cache pool fills up.  If that happens then Docker content would write to the array, then when mover executes, it would move from array to cache pool (again, provided there is space on the cache). 

 

So with this config, I start up two Docker containers, and immediately I hear the physical disks (spindle drives) working every 4 seconds or so.  If I stop the Docker containers, the physical disks are quiet and I don't hear them.

 

I would think if I have Docker shares set up to "Prefer: Cache" and cache pool is nearly empty (confirmed during testing cache it had over 900GB of free space), there would be no reason for Docker to write to or access the physical disks. 

 

Is everything set up properly and this is the way it is supposed to work?  Or do I need to change some settings somewhere?  Appreciate any suggestions.  Thanks!

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Hi @trurl, thanks for the suggestion.  I have resolved the issue.  😁

 

I reviewed the diagnostics and everything looked OK, so thought I'd try a reboot.  Rebooting ran the Fix Common Problems Community App (thanks @Squid!), which informed me Share system is set to cache only, but files / folders exist on the array.  I corrected by following the steps in this topic: 

 

After running mover (I had logging enabled to confirm which files it moved), I then set the share to Only: Cache.  Checked mover log and confirmed it moved the docker.img and a vm image file to the cache drive.  This issue arose because I set up Unraid without a cache at first.. set up docker.. then added cache NVME pool.  After doing that somehow the docker/VM images files got "stuck" on the array, so whenever I would run a Docker app it would access the docker.img file on the array, and I would hear the drives being pinged every 3-4 seconds.

 

Now the docker/VM content is properly stored on the cache pool & I have set the docker/VM shares properly to use only the cache.  And when I run docker, I hear nothing but SILENCE!  Thanks for the suggestion of using the diagnostics, as it did tell me much more about the system and the way it is configured.  And it led me to the eventual solution!

 

I also changed the "Fix Common Problems" app to run every day.  That way if something weird occurs I don't have to wait for a reboot to find it.  (As before the app was set to only run on bootup).  Thanks all.

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  • Solution
33 minutes ago, tropical b said:

somehow the docker/VM images files got "stuck" on the array, so whenever I would run a Docker app it would access the docker.img file on the array,

Because 

On 4/30/2022 at 10:08 PM, trurl said:

Nothing can move open files

These are open by Docker and VM Manager, which must be disabled in Settings so the img can be moved 

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