docs: Update documentation on balloon device

Add notes around known limitations of the balloon device

Signed-off-by: Jack Thomson <[email protected]>

Author: JackThomson2

docs/ballooning.md

@@ -263,3 +263,15 @@ cannot be enabled later by providing a `polling_interval` non-zero value.
 Furthermore, if the balloon was configured with statistics pre-boot through a
 non-zero `stats_polling_interval_s` value, the statistics cannot be disabled
 through a `polling_interval` value of zero post-boot.
+
+## Notes
+
+- Firecracker has no control around the speed of inflation or deflation, this is
+  dictated by the guest kernel. As a result, if the balloon is left inflated the
+  host could see serverly affected performance, it is therefore recommended to
+  only have the balloon inflated for shorter periods of time to reclaim memory.
+
+- The `deflate_on_oom` flag which allows the balloon to deflate in an out of
+  memory scenario, if it has yet to reach its target size will continue to try
+  and inflate even after releasing memory. This can lead to high CPU usage with
+  a high allocations and deallocations.