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Re: [Qemu-block] [Qemu-devel] [PATCH v2] file-posix: add drop-cache=on|o

From: Stefan Hajnoczi
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2] file-posix: add drop-cache=on|off option
Date: Wed, 6 Mar 2019 10:18:11 +0000
User-agent: Mutt/1.11.3 (2019-02-01)

On Fri, Mar 01, 2019 at 10:30:07AM -0600, Eric Blake wrote:
> On 3/1/19 10:09 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.
> > 
> > Suggested-by: Neil Skrypuch <address@hidden>
> > Tested-by: Neil Skrypuch <address@hidden>
> > Reviewed-by: Stefano Garzarella <address@hidden>
> > Reviewed-by: Eric Blake <address@hidden>
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > ---
> > v2:
> >  * Remove outdated comment about libvirt feature detection [danpb]
> Question - if we used qapi's 'if':COND to only declare the field on
> platforms where we know at compile time that we can support it, would
> that be enough for libvirt to introspect that if the field exists then
> migration is safe, without having to rely on an query-qemu-features command?

Yes, although this raises another question:

The drop-cache implementation is not #ifdefed in file-posix.c.  If we
make the QMP schema conditional, should we also #ifdef the command-line
option in raw_runtime_opts[] to prevent QEMU from silently ignoring this


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