[Qemu-devel] qemu disk use writeback don't take effect in centos 7.2 1511

[Paul Schlacter]

qemu version:  2.9.0
image: centos 7.2 1511 raw
cache mode: writeback


In centos7.2 time, the use of writeback and none, the result of dd disk is
the same io, no change

But I replaced centos7.1 centos7.3 centos7.4 when writeback was
significantly faster than none


Would like to ask what is going on, how to solve ?