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Re: [Qemu-devel] [PATCH v8 10/14] migration: add postcopy migration of d

From: John Snow
Subject: Re: [Qemu-devel] [PATCH v8 10/14] migration: add postcopy migration of dirty bitmaps
Date: Thu, 16 Nov 2017 19:36:07 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/16/2017 05:24 AM, Vladimir Sementsov-Ogievskiy wrote:
>>> +    /* if a block is zero we need to flush here since the network
>>> +     * bandwidth is now a lot higher than the storage device bandwidth.
>>> +     * thus if we queue zero blocks we slow down the migration. */
>> Can you elaborate on this for me?
> it comes from migration/block.c, as it was a prototype for dirty bitmaps
> migration.
> May be the original thought was to give destination storage more time to
> handle
> write-zeros? It may make sense if it can't do it fast but really writes
> zeros.
>>> +    if (flags & DIRTY_BITMAP_MIG_FLAG_ZEROES) {
>>> +        qemu_fflush(f);
>>> +    } else {
>>> +        qemu_put_be64(f, buf_size);
>>> +        qemu_put_buffer(f, buf, buf_size);
>>> +    }
>>> +
>>> +    g_free(buf);
>>> +} 

Peter, Stefan; do you have any insight on the reason for this blurb's
appearance in migration/block.c? Looks like it showed up in

I'm not sure I understand what the concern was that lead to this, unless
it's simply that without a flush() we can't guarantee progress.

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