[CRIU] why page-server does not transfer all the dumped pages?

[Pavel Emelyanov]

On 04/28/2015 06:35 PM, Hui Kang wrote:
> 
> On Tue, Apr 28, 2015 at 11:28 AM, Pavel Emelyanov <xemul at parallels.com <mailto:xemul at parallels.com>> wrote:
> 
>     On 04/28/2015 06:23 PM, Hui Kang wrote:
>     > Hi,
>     > I have tested about the disk-less page migrations. As the instruction says, after dumping
>     > pages to the destination, there exists some small pages on the source node.
>     >
>     > I am just curious what is the rational of not transferring all the dumped pages. Thanks.
> 
>     If you request for --page-server dump (or pre-dump) no pages are put in
>     the source images. Do you see non-empty pages*.img files?
> 
> 
> I follow the instructions on this link (http://criu.org/Disk-less_migration). Step 4 says "As was
> said, CRIU images other than those with pages are very small and this won't take long. "
> I am asking why those CRIU images are not sent to the page-server.

Ah, I see.

The idea behind this is quite simple. In order to transfer images as they are generated requires
a non-trivial protocol for it. We didn't want to implement it since there exists plenty of them.
And sine images are typically several hundreds kilos in size, we can just dump them into tmpfs, 
then scp (or rsync or whatever) into the destination side. With pages this trick doesn't work for
two reasons. First, pages images can occupy gigs, we can't put them on tmpfs. Second -- while
doing iterations we need to drop the overwritten pages from images, thus the transferring protocol
is not just "copy". So, as a balance, we implemented the protocol for pages only.

If you have ideas how to improve that and transfer all images w/o intermediate storage (even tmpfs)
you're welcome to share it :)

-- Pavel