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Re: [Qemu-devel] [PATCH v1] migration: skip sending ram pages released b

From: Roman Kagan
Subject: Re: [Qemu-devel] [PATCH v1] migration: skip sending ram pages released by virtio-balloon driver.
Date: Thu, 10 Mar 2016 12:49:13 +0300
User-agent: Mutt/1.5.24 (2015-08-30)

On Fri, Mar 04, 2016 at 02:32:47PM +0530, Jitendra Kolhe wrote:
> Even though the pages which are returned to the host by virtio-balloon
> driver are zero pages, the migration algorithm will still end up
> scanning the entire page ram_find_and_save_block() -> ram_save_page/
> ram_save_compressed_page -> save_zero_page() -> is_zero_range().  We
> also end-up sending some control information over network for these
> page during migration. This adds to total migration time.

I wonder if it is the scanning for zeros or sending the whiteout which
affects the total migration time more.  If it is the former (as I would
expect) then a rather local change to is_zero_range() to make use of the
mapping information before scanning would get you all the speedups
without protocol changes, interfering with postcopy etc.


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