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[Qemu-commits] [qemu/qemu] f357fc: file-posix: add drop-cache=on|off opt

From: Peter Maydell
Subject: [Qemu-commits] [qemu/qemu] f357fc: file-posix: add drop-cache=on|off option
Date: Thu, 14 Mar 2019 10:45:35 +0000 (UTC)

  Branch: refs/heads/master
  Home:   https://github.com/qemu/qemu
  Commit: f357fcd890a8d6ced6d261338b859a41414561e9
  Author: Stefan Hajnoczi <address@hidden>
  Date:   2019-03-13 (Wed, 13 Mar 2019)

  Changed paths:
    M block/file-posix.c
    M qapi/block-core.json

  Log Message:
  file-posix: add drop-cache=on|off option

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>
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>
Message-id: address@hidden
Message-Id: <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>

  Commit: dbbc277510aa39ea0b7658479e4d67779dede0ea
  Author: Peter Maydell <address@hidden>
  Date:   2019-03-14 (Thu, 14 Mar 2019)

  Changed paths:
    M block/file-posix.c
    M qapi/block-core.json

  Log Message:
  Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into 

Pull request

 * Add 'drop-cache=on|off' option to file-posix.c.  The default is on.
   Disabling the option fixes a QEMU 3.0.0 performance regression when live
   migrating on the same host with cache.direct=off.

# gpg: Signature made Wed 13 Mar 2019 11:07:48 GMT
# gpg:                using RSA key 9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <address@hidden>" [full]
# gpg:                 aka "Stefan Hajnoczi <address@hidden>" [full]
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  file-posix: add drop-cache=on|off option

Signed-off-by: Peter Maydell <address@hidden>

Compare: https://github.com/qemu/qemu/compare/abb7ede1fb60...dbbc277510aa

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