On Tue, Aug 29, 2017 at 10:12:58AM -0300, Eduardo Habkost wrote: > On Tue, Aug 29, 2017 at 12:13:45PM +0100, Daniel P. Berrange wrote: > > On Thu, Aug 24, 2017 at 04:23:15PM -0300, Eduardo Habkost wrote: [...] > > > @@ -4172,6 +4172,9 @@ the path to either a shared memory or huge page > > > filesystem mount. > > > The @option{share} boolean option determines whether the memory > > > region is marked as private to QEMU, or shared. The latter allows > > > a co-operating external process to access the QEMU memory region. > > > +Setting the @option{discard-data} boolean option to @var{on} > > > +indicates that file contents can be destroyed when QEMU exits, > > > +to avoid unnecessarily flushing data to the backing file. > > > > We should note that this only works if QEMU shuts down normally. If QEMU > > is aggressively killed (SIGKILL) or aborts for some reason, then we'll > > never get a chance to invoke madvise(), so presumably the kernel will > > still flush the data > > Good point. I tried to not give any guarantees by saying > "contents _can_ be destroyed", but users may still have different > expectations. > > I will change it to: > > Setting the @option{discard-data} boolean option to @var{on} > indicates that file contents can be destroyed when QEMU exits, > to avoid unnecessarily flushing data to the backing file. Note > that @option{discard-data} is only an optimization, and QEMU > might not discard file contents if it aborts unexpectedly or is > terminated using SIGKILL.
Fixup patch: Signed-off-by: Eduardo Habkost <ehabk...@redhat.com> --- qemu-options.hx | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/qemu-options.hx b/qemu-options.hx index ad985e4..de9a18a 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4174,7 +4174,10 @@ region is marked as private to QEMU, or shared. The latter allows a co-operating external process to access the QEMU memory region. Setting the @option{discard-data} boolean option to @var{on} indicates that file contents can be destroyed when QEMU exits, -to avoid unnecessarily flushing data to the backing file. +to avoid unnecessarily flushing data to the backing file. Note +that @option{discard-data} is only an optimization, and QEMU +might not discard file contents if it aborts unexpectedly or is +terminated using SIGKILL. @item -object rng-random,id=@var{id},filename=@var{/dev/random} -- 2.9.4