Re: How to optimize pre-copy algorithm of xen to minimize downtime?

[Mats Petersson <mats.petersson@citrix.com>]

On 11/12/12 14:33, digvijay chauhan wrote:
>
> Hello,
>          If I want to optimize the performance of precopy algorithm so 
> that live migration of virtual machine using xen occurs with minimum 
> downtime,then how to do it?
Having spent quite a bit of time studying the time that a domain is down 
during migration in XenServer, I'm not convinced pre-copy is what is the 
biggest part of downtime, assuming the Open Source product behaviour is 
at least mostly doing the same things - although what the host and guest 
is up to before it gets suspended/paused will have a big impact as well. 
If the guest is VERY busy, you may well end up with very little memory 
that doesn't need to be copied in the final copy - not sure how you can 
improve this.

Have you made some measurements to show where the time is spent during 
downtime.
What workloads are you looking at? What downtime numbers have you got at 
present, and what portion of this is down to actually copy time, and 
what is other things that need to happen during migration (e.g. bringing 
the virtual HD and Net devices down, and then up again on the new guest)?

My above comments assume there is a FAST link between the old and new 
host (my testing has been primarily using "localhost" migration, so the 
new guest is on the same machine as the old one). For a slow link, you 
may find that the copy time is more of the whole time, but again, I'm 
not sure there is a huge amount that can be done without a rather large 
amount of added complexity - and that may well be better put elsewhere, 
e.g. compression on the network link, or some such.

I have patch for Linux 3.7rcX that improves the time Dom0 spends mapping 
in the guest memory, which does have a good impact on the overall copy 
time, but not that huge impact on the downtime (since the copy time is 
only small part of overall downtime, at least in my testcases). Look at 
list archives for Friday 7th of Dec for the latest version.

--
Mats
>
> regards,
> Digvijay
>