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Re: [Qemu-block] [Qemu-devel] migration: add incremental drive-mirror an

From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] migration: add incremental drive-mirror and blockdev-mirror with dirtymap
Date: Tue, 9 May 2017 10:48:21 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.0

On 05/09/2017 03:35 AM, Daniel Kučera wrote:
>     Hm, I suppose that's right, pending cache issues, perhaps?
>     (1) Write occurs; cached
>     (2) Bitmap is added
>     (3) Write occurs, cached
>     (4) ZFS snapshot is taken
>     (5) Data is flushed to backing storage.
>     Now, the ZFS snapshot is migrated, but is missing the writes that
>     occurred in (1) and (3).
>     Next, we mirror the data in the bitmap, but it only includes the data
>     from (3).
>     The target now appears to be missing the write from (1) -- maybe,
>     depending on how the volume snapshot occurs.
> Yes, that's why I'm using cache=none. Libvirt doesn't allow you to
> migrate VM which uses drive cache anyway (unless you specify flag unsafe).

It will be important to document the exact use cases in which this mode
is "supported" and the shortcomings and cases in which it is not supported.

The final version should include a test and some documentation, but I
think the feature is workable once we pay attention to error conditions.



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