Re: [Qemu-devel] [regression] Clock jump on VM migration

[Stefan Hajnoczi]

On Tue, Feb 12, 2019 at 10:56:58AM +0800, Stefan Hajnoczi wrote:
> I'll return to this issue on Monday.

I have played around with posix_fadvise(POSIX_FADV_DONTNEED) and
measured 100+ millisecond latencies.  This is not a great primitive to
rely on during migration downtime since it can be slow.

On the other hand, we need it to ensure consistency when migrating to
another host with cache.direct=off.  In the cross-host migration case
the invalidation will be relatively fast because few pages will be
present.

An asynchronous flush and page cache invalidation on the source host at
the beginning of live migration would help your use case, but I don't
think the complexity of adding it is worthwhile (it doesn't help the
cross-host migration case).

Let's go back to the original idea of adding a parameter to disable the
flush and page cache invalidation.  I will send a patch.

Stefan

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