[libvirt] Cluster_size parameter issue on qcow2 image format

I have been working o the qcow2 image format , I theory regarding cluster size it is written that as the size of cluster increase performance should increase. But something surprising happen The performance is degrading as the size of cluster increased from 64K to 1M ( during expansion of qcow2 image) can any one tell why ? DISCLAIMER: ----------------------------------------------------------------------------------------------------------------------- The contents of this e-mail and any attachment(s) are confidential and intended for the named recipient(s) only. It shall not attach any liability on the originator or NECHCL or its affiliates. Any views or opinions presented in this email are solely those of the author and may not necessarily reflect the opinions of NECHCL or its affiliates. Any form of reproduction, dissemination, copying, disclosure, modification, distribution and / or publication of this message without the prior written consent of the author of this e-mail is strictly prohibited. If you have received this email in error please delete it and notify the sender immediately. . -----------------------------------------------------------------------------------------------------------------------