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And also change links from old documentation to docs sub-directory. Fixes #444 Signed-off-by: bin liu <bin@hyper.sh>
48 lines
2.2 KiB
Markdown
48 lines
2.2 KiB
Markdown
# What Is VMCache and How To Enable It
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* [What is VMCache](#what-is-vmcache)
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* [How is this different to VM templating](#how-is-this-different-to-vm-templating)
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* [How to enable VMCache](#how-to-enable-vmcache)
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* [Limitations](#limitations)
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### What is VMCache
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VMCache is a new function that creates VMs as caches before using it.
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It helps speed up new container creation.
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The function consists of a server and some clients communicating
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through Unix socket. The protocol is gRPC in [`protocols/cache/cache.proto`](../../src/runtime/protocols/cache/cache.proto).
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The VMCache server will create some VMs and cache them by factory cache.
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It will convert the VM to gRPC format and transport it when gets
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requested from clients.
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Factory `grpccache` is the VMCache client. It will request gRPC format
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VM and convert it back to a VM. If VMCache function is enabled,
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`kata-runtime` will request VM from factory `grpccache` when it creates
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a new sandbox.
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### How is this different to VM templating
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Both [VM templating](../how-to/what-is-vm-templating-and-how-do-I-use-it.md) and VMCache help speed up new container creation.
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When VM templating enabled, new VMs are created by cloning from a pre-created template VM, and they will share the same initramfs, kernel and agent memory in readonly mode. So it saves a lot of memory if there are many Kata Containers running on the same host.
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VMCache is not vulnerable to [share memory CVE](../how-to/what-is-vm-templating-and-how-do-I-use-it.md#what-are-the-cons) because each VM doesn't share the memory.
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### How to enable VMCache
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VMCache can be enabled by changing your Kata Containers config file (`/usr/share/defaults/kata-containers/configuration.toml`,
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overridden by `/etc/kata-containers/configuration.toml` if provided) such that:
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* `vm_cache_number` specifies the number of caches of VMCache:
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* unspecified or == 0
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VMCache is disabled
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* `> 0`
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will be set to the specified number
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* `vm_cache_endpoint` specifies the address of the Unix socket.
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Then you can create a VM templating for later usage by calling:
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```
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$ sudo kata-runtime factory init
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```
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and purge it by `ctrl-c` it.
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### Limitations
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* Cannot work with VM templating.
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* Only supports the QEMU hypervisor.
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