Re: [Qemu-devel] [PATCH] file-posix: add drop-cache=on|off option

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Feb 27, 2019 at 03:56:27PM +0000, Daniel P. Berrangé wrote:
> On Wed, Feb 27, 2019 at 03:33:11PM +0000, Stefan Hajnoczi wrote:
> > 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).
> 
> The commit message said that libvirt would use this new option to
> detect availability of the cache drop feature. That should probably
> be removed from the commit message, as this caveat about non-portable
> impl means libvirt can't actually rely on it.

Okay, will fix.

Stefan

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