From b77a66fc7854e6aaba82395cca31a13b7c2c0f51 Mon Sep 17 00:00:00 2001 From: George Dunlap Date: Thu, 19 Mar 2015 17:09:19 +0000 Subject: [PATCH] docs: Mention a common pitfall in ballooning Several users have reported that their available free memory in the guest when they used maxmem >> memory was much smaller than when maxmem == memory. This is the unavoidable consequence of how ballooning works, but it's not something users expect. Warn them of this effect in the place we tell them how to make it happen, so they aren't surprised. Signed-off-by: George Dunlap Acked-by: Ian Campbell --- docs/man/xl.cfg.pod.5 | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5 index ec28530527..f936dfc5ff 100644 --- a/docs/man/xl.cfg.pod.5 +++ b/docs/man/xl.cfg.pod.5 @@ -264,6 +264,13 @@ if the values of B and B differ. A "pre-ballooned" HVM guest needs a balloon driver, without a balloon driver it will crash. +NOTE: Because of the way ballooning works, the guest has to allocate +memory to keep track of maxmem pages, regardless of how much memory it +actually has available to it. A guest with maxmem=262144 and +memory=8096 will report significantly less memory available for use +than a system with maxmem=8096 memory=8096 due to the memory overhead +of having to track the unused pages. + =back =head3 Guest Virtual NUMA Configuration -- 2.39.5