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Re: [Qemu-block] [Qemu-devel] [PATCH] file-posix: add drop-cache=on|off
From: |
Stefano Garzarella |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH] file-posix: add drop-cache=on|off option |
Date: |
Wed, 27 Feb 2019 10:57:55 +0100 |
User-agent: |
NeoMutt/20180716 |
On Tue, Feb 26, 2019 at 03:35:49PM +0000, 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.
>
> Suggested-by: Neil Skrypuch <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> qapi/block-core.json | 5 +++++
> block/file-posix.c | 14 ++++++++++++++
> 2 files changed, 19 insertions(+)
>
Reviewed-by: Stefano Garzarella <address@hidden>