Re: [Qemu-devel] [RFC] lively write vmstate with predictable size

[Juan Quintela <quintela@redhat.com>]

Wenchao Xia <xiawenc@linux.vnet.ibm.com> wrote:
> resent the mail to mail-list.
> -------------------
>
> Hi, Paolo and Juan
>     Currently savevm needs pause vm, and I am working on that make it
>  lively. Considering the flexibility I'd like to split out the
>  functions apart as following:
>  1) snapshot lively internal/external
>  2) save vmstate lively internal/external
>  3) assemble them as will
>
>  1) was sent at
>  http://lists.nongnu.org/archive/html/qemu-devel/2012-12/msg02393.html
>
>  but for 2), I think it have problem because file size may grow to a size
>  out of control. Considering the migration code, I'd like to propose a
>  way to fix it as following:
>
>  Migration logic:
>  Src send->dest recv->data analysis->copy data
>  Savevm logic:
>  Src send->write data to qcow2.
>
> My suggestion:
>  Savevm logic:
>  Src send->dest recv->data analysis->write data to qcow2/external with
>  addr.
>
>    The idea is do the write operation after data analysis, and overwrite
>  old data if address overlaps. So this will need qcow2 support
>  write snapshot data at "address", and also change some savevm logic.

We could change the code to do it, but it is not going to be
trivial. Furthermore we should disable xbzrle (in this case, it just
makes no sense)

The easiest way that I can think of is changing the
arch_init.c:

We can:
a- reserve a big enough area in the qcow2 image/external to store the
whole ram
b- change ram_save_block() on that file to directly write to the "right"
   position inside the image.
c- change ram_load() to understand the new format.

Notice that this would have to be a different implementation that
sending data over tcp, as there we want (something) similar to the
current code.


>    Could u give some some comments on this to see if it is workable?

It needs lots of code rearragement as far as I can think, it is doable
but it is quite a bit of work.

Later, Juan.