Merge pull request #5289 from robhoes/add-docs

More docs on the new site

doc/content/squeezed/_index.md

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+title = "Squeezed"
+weight = 50
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+
+Squeezed is the XAPI Toolstack's host memory manager (aka balloon driver).
+Squeezed uses ballooning to move memory between running VMs, to avoid wasting
+host memory.
+
+Principles
+----------
+
+1. Avoid wasting host memory: unused memory should be put to use by returning
+   it to VMs.
+2. Memory should be shared in proportion to the configured policy.
+3. Operate entirely at the level of domains (not VMs), and be independent of
+   Xen toolstack.

doc/content/squeezed/architecture/index.md

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+title = "Architecture"
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+
+Squeezed is responsible for managing the memory on a single host. Squeezed
+"balances" memory between VMs according to a policy written to Xenstore.
+
+The following diagram shows the internals of Squeezed:
+
+![Internals of squeezed](squeezed.png)
+
+At the center of squeezed is an abstract model of a Xen host. The model
+includes:
+
+- The amount of already-used host memory (used by fixed overheads such as Xen
+  and the crash kernel).
+- Per-domain memory policy specifically `dynamic-min` and `dynamic-max` which
+  together describe a range, within which the domain's actual used memory
+  should remain.
+- Per-domain calibration data which allows us to compute the necessary balloon
+  target value to achive a particular memory usage value.
+
+Squeezed is a single-threaded program which receives commands from xenopsd over
+a Unix domain socket. When Xenopsd wishes to start a new VM, squeezed will be
+asked to create a "reservation". Note this is different to the Xen notion of a
+reservation. A squeezed reservation consists of an amount of memory squeezed
+will guarantee to keep free labelled with an id. When Xenopsd later creates the
+domain to notionally use the reservation, the reservation is "transferred" to
+the domain before the domain is built.
+
+Squeezed will also wake up every 30s and attempt to rebalance the memory on a
+host. This is useful to correct imbalances caused by balloon drivers
+temporarily failing to reach their targets. Note that ballooning is
+fundamentally a co-operative process, so squeezed must handle cases where the
+domains refuse to obey commands.
+
+The "output" of squeezed is a list of "actions" which include:
+
+- Set domain x's `memory/target` to a new value.
+- Set the `maxmem` of a domain to a new value (as a hard limit beyond which the
+  domain cannot allocate).
+