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/var/lib/nova/instances RAM cache usage

asked 2016-07-30 20:45:06 -0500

dzrwork1 gravatar image

updated 2016-08-01 14:01:31 -0500

I've been looking at RAM allocation on the compute hosts.

Using a tool, I see that In a number of environments, I have /var/lib/nova/instances using a lot of RAM cache - to the tune of 119G (another env is using 135 GB).

The /var/lib/nova/instances/'instanceID'/disk directories are 49 GB total (3 files). The files inside of the _base directory is using 68 G. So, between those three, 117 of 119 GB of cache are being used.

Could _base be told not to sit in cache? Would just just impact VM startup (which I don't care about so much?) I'm not sure how I could get there, though.

The OS ends up using Swap. I'm wondering if the OS doesn't want to remove the above data from cache. If not the system using swap, I wouldn't care about the cache usage.

In another environment,/var/lib/nova/instances is using only 628K of cache.

We have id=drive-virtio-disk0,format=qcow2,cache=none set, so why is /var/lib/nova/instances/'instanceID'/disk cached. Maybe a qemu bug in the version I'm running? I just posted to the qemu mailing list.

That doesn't account for the _base directory files being cached.

Recall, that neither of these locations are cached in another environment I have.. which is -supposed to be- configured in parallel.

We're using Icehouse and can't change now.


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answered 2016-08-03 13:46:52 -0500

dzrwork1 gravatar image

After all that -- a disk backup process ended up triggering the read of all those files into cache. The environment which doesn't have those files in cache, doesn't have the backup process running.


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Asked: 2016-07-30 20:45:06 -0500

Seen: 206 times

Last updated: Aug 03 '16