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[Qemu-discuss] Domain state file size and live migration

From: Vincent KHERBACHE
Subject: [Qemu-discuss] Domain state file size and live migration
Date: Mon, 23 Feb 2015 19:06:54 +0100
User-agent: Amstrad CPC 464

Hi all,

I want to be able to retrieve the memory used by a QEMU domain in order
to estimate the amount of data that will be sent during a live migration.

I'm using a shared storage on a dedicated network to store all base
image (raw) and backing images (qcow2) of VMs.
So, if I understood correctly, by doing a live migration we should only
transfer the *memory used* by the guest (also the dirty pages) and its
cpu's state.

(migration cmd: "virsh --connect qemu+tcp://src_host/system migrate
--live --verbose domain qemu+tcp://dst_host/system")

The problem is that, by monitoring the network, I saw that almost 1GiB
are sent between the two hosts, but the actual memory usage of the VM
(using free, top and vmstat cmds from guest) indicates that only 150MiB
are actually used (idle VM).
Also, when I do a "virsh save", the domain state file is indeed about
1GiB, not 150MiB.

I tried 'dominfo', 'dumpxml', 'dommmemstat' cmds (also the 'virt-top'
tool) to get more detailed informations from hypervisor but they only
show the *total memory allocated* to the VM (which is 4GiB).

So, I'm trying to figure out what contains exactly the domain state file
to understand what are the 'extra data' sent between the two hosts.

Btw, is there a way to get the actual memory used by a running domain
from QEMU ?

Thanks for your help.



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