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

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Commit dd577a26ff ("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 <neil@tembosocial.com>
Tested-by: Neil Skrypuch <neil@tembosocial.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20190307164941.3322-1-stefanha@redhat.com
Message-Id: <20190307164941.3322-1-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2019-03-07 16:49:41 +00:00
parent 3f3bbfc7ce
commit f357fcd890
2 changed files with 22 additions and 0 deletions
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6
qapi/block-core.json
View file

@ -2816,6 +2816,10 @@
# @locking: whether to enable file locking. If set to 'auto', only enable
# when Open File Descriptor (OFD) locking API is available
# (default: auto, since 2.10)
# @drop-cache: invalidate page cache during live migration. This prevents
# stale data on the migration destination with cache.direct=off.
# Currently only supported on Linux hosts.
# (default: on, since: 4.0)
# @x-check-cache-dropped: whether to check that page cache was dropped on live
# migration. May cause noticeable delays if the image
# file is large, do not use in production.
@ -2828,6 +2832,8 @@
'*pr-manager': 'str',
'*locking': 'OnOffAuto',
'*aio': 'BlockdevAioOptions',
'*drop-cache': {'type': 'bool',
'if': 'defined(CONFIG_LINUX)'},
'*x-check-cache-dropped': 'bool' } }
##