On Tue, Feb 26, 2019 at 10:54:55AM -0600, Eric Blake wrote:
> On 2/26/19 9:35 AM, Stefan Hajnoczi wrote:
> > Commit dd577a26ff03b6829721b1ffbbf9e7c411b72378 ("block/file-posix:
> > implement bdrv_co_invalidate_cache() on Linux") introduced page cache
> > invalidation so that cache.direct=off live migration is safe on Linux.
> >
> > The invalidation takes a significant amount of time when the file is
> > large and present in the page cache. Normally this is not the case for
> > cross-host live migration but it can happen when migrating between QEMU
> > processes on the same host.
> >
> > On same-host migration we don't need to invalidate pages for correctness
> > anyway, so an option to skip page cache invalidation is useful. I
> > investigated optimizing invalidation and detecting same-host migration,
> > but both are hard to achieve so a user-visible option will suffice.
> >
> > As a bonus this option means that the cache invalidation feature will
> > now be detectable by libvirt via QMP schema introspection.
>
> Do you still want to pursue the QMP query-qemu-features command, or does
> this delay that for another day?The presence of this new option doesn't guarantee that dropping caches works. It is currently only implemented on Linux. We still need query-qemu-features so that libvirt can detect whether this QEMU binary can drop caches (e.g. on Linux vs FreeBSD). Stefan
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