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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH] file-posix: add drop-cache=on|off option
Date: Wed, 27 Feb 2019 15:33:11 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

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).


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