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

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

Hi, Juan
  Thank u for reviewing on this, have some questions below.

> 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.
> 
  I agree with your way for that it saves computation for
data analysis. But why a big area should be reserved and new format
is needed? I thought every thing would be same as old vmstate save
case with vm stopped, if qemu can overwrite the contents in the right
place and leaves remaining tag unchanged.

  how about write a new function replace save_block_hdr() and following
buffer writing, which would be an abstract function used to write data
with addr? It still encapsulate it as save_block_hdr() logic, but in
savevm case it will directly write data to a right place. In this way
at vmstate contents level, nothing changes, it still stores the same
stream as if vm is stopped.

> 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.
> 
  I think it would be a choice between performance and code unifying,
personally I guess performance is the first to consider for qemu.

> 
>>     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.
> 


-- 
Best Regards

Wenchao Xia