About Libvirt Setmem&dommemstat Function

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I am using 'virsh setmem' to adjust the vm memory. But I don't know what is the lowerlimit memory that can be adjusted to, so I try to update the version of libvirt&qemu to get caches of the vm memory so that I can calculate the lowerlimit. And when I upgrade libvirt to 4.10.0, qemu upgrade to 4.1.0, still, I cannot get 'stat-disk-caches'.  How can I solve it ? Thx.
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