From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:34271) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gyg0g-0003Ez-61 for qemu-devel@nongnu.org; Tue, 26 Feb 2019 11:55:16 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gyg0c-00065M-FB for qemu-devel@nongnu.org; Tue, 26 Feb 2019 11:55:12 -0500 References: <20190226153549.17867-1-stefanha@redhat.com> From: Eric Blake Message-ID: <8a8d76d1-1280-f3d9-3d72-1c662d1ad666@redhat.com> Date: Tue, 26 Feb 2019 10:54:55 -0600 MIME-Version: 1.0 In-Reply-To: <20190226153549.17867-1-stefanha@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] file-posix: add drop-cache=on|off option List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi , qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, Max Reitz , Markus Armbruster , Kevin Wolf , Neil Skrypuch , Peter Krempa 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 > Signed-off-by: Stefan Hajnoczi > --- > qapi/block-core.json | 5 +++++ > block/file-posix.c | 14 ++++++++++++++ > 2 files changed, 19 insertions(+) > Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org