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howto: Add VMCache introduction and guide

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Add "What Is VMCache and How To Enable It" to howto.

Fixes: #392

Signed-off-by: Hui Zhu <teawater@hyper.sh>
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Hui Zhu 2019-03-07 13:03:47 +08:00
parent a2fe01131d
commit 7b94750180
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# What Is VMCache and How To Enable It
* [What is VMCache](#what-is-vmcache-and-how-to-enable-it)
* [How is this different to VM templating](#how-is-this-different-to-vm-templating)
* [How to enable VMCache](#how-to-enable-vmcache)
* [Limitations](#limitations)
### What is VMCache
VMCache is a new function that creates VMs as caches before using it.
It helps speed up new container creation.
The function consists of a server and some clients communicating
through Unix socket. The protocol is gRPC in [protocols/cache/cache.proto](https://github.com/kata-containers/runtime/blob/master/protocols/cache/cache.proto).
The VMCache server will create some VMs and cache them by factory cache.
It will convert the VM to gRPC format and transport it when gets
requested from clients.
Factory grpccache is the VMCache client. It will request gRPC format
VM and convert it back to a VM. If VMCache function is enabled,
kata-runtime will request VM from factory grpccache when it creates
a new sandbox.
### How is this different from VM templating
Both [VM templating](https://github.com/kata-containers/documentation/blob/master/how-to/what-is-vm-templating-and-how-do-I-use-it.md) and VMCache help speed up new container creation.
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.
VMCache is not vulnerable to [share memory CVE](https://github.com/kata-containers/documentation/blob/master/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.
### How to enable VMCache
VMCache can be enabled by changing your Kata Containers config file (`/usr/share/defaults/kata-containers/configuration.toml`,
overridden by `/etc/kata-containers/configuration.toml` if provided) such that:
* `vm_cache_number` specifies the number of caches of VMCache:
* unspecified or == 0
VMCache is disabled
* `> 0`
will be set to the specified number
* `vm_cache_endpoint` specifies the address of the Unix socket.
Then you can create a VM templating for later usage by calling:
```
$ sudo kata-runtime factory init
```
and purge it by ctrl-c it.
### Limitations
* Cannot work with VM templating.
* Cannot work with vsock.
* Only supports the qemu hypervisor.

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how-to/what-is-vm-templating-and-how-do-I-use-it.md
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@ -7,6 +7,11 @@ by cloning from a pre-created template VM, and they will share the
same initramfs, kernel and agent memory in readonly mode. It is very
much like a process fork done by the kernel but here we *fork* VMs.
### How is this different from VMCache
Both [VMCache](https://github.com/kata-containers/documentation/blob/master/how-to/what-is-vm-cache-and-how-do-I-use-it.md) and VM templating help speed up new container creation.
When VMCache enabled, new VMs are created by the VMCache server. So it is not vulnerable to share memory CVE because each VM doesn't share the memory.
VM templating saves a lot of memory if there are many Kata Containers running on the same host.
### What are the Pros
VM templating helps speed up new container creation and saves a lot
of memory if there are many Kata Containers running on the same host.