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

[Eric Blake <eblake@redhat.com>]

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?

> 
> Suggested-by: Neil Skrypuch <neil@tembosocial.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  qapi/block-core.json |  5 +++++
>  block/file-posix.c   | 14 ++++++++++++++
>  2 files changed, 19 insertions(+)
> 
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org