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[Qemu-devel] What's the advantages of POSTCOPY over CPU-THROTTLE?


From: Zhangbo (Oscar)
Subject: [Qemu-devel] What's the advantages of POSTCOPY over CPU-THROTTLE?
Date: Wed, 6 Jan 2016 09:41:15 +0000

Hi all:
  Postcopy is suitable for migrating guests which have large page change rates. 
It 
    1 makes the guest run at the destination ASAP.
    2 makes the downtime of the guest small enough.
    If we don't take the 1st advantage into account, then, its benefit seems 
similar with CPU-THROTTLE: both of them make the guest's downtime small during 
migration.
 
    CPU-THROTTLE would make the guest's dirtypage rate *smaller than the 
network bandwidth*, in order to make the to_send_page_number in each iteration 
convergent and achieve the small-enough downtime during the last iteration.
    If we adopt POST-COPY here, the guest's dirtypage rate would *become equal 
to the bandwidth*, because we have to fetch its memory from the source side, 
via the network.
    Both of them would introduce performance degradations of the guest, which 
may in turn cause downtime larger.

    So, here comes the question: If we just compare POSTCOPY with CPU-THROTTLE 
for their advantages in decreasing downtime, POSTCOPY seems has no pos over 
CPU-THROTTLE, is that right?

    Meanwhile, Are there any other benifits of POSTCOPY besides the 2 mentioned 
above?

Oscar

    


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