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You can run Dockers inside of Linux containers with virtually no performance impact.
Where I use LXC is for long running systems and creating isolation density within the Cloud. For example, a bunch of LXC's inside of an Azure VM which further separate my Docker apps btw PreProd and Production.
Which one to learn first? If you want to really understand things, learn LXC and you'll know Docker. It's like starting with DOS then learning Windows.
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