http://itscblog.tamu.edu/improve-disk-io-performance-in-kvm/
기존 qcow2 이미지 preallocation=metadat 옵션주고 convert
# qemu-img convert -f qcow2 -O qcow2 -o preallocation=metadata centos_test0_nopreallc.qcow2 centos_test0_preallc.qcow2
Preallocation mode (allowed values: off, metadata).
An image with preallocated metadata is initially larger but can improve performance when the image needs to grow.
# qemu-img create -f qcow2 -o size=200G,preallocation=metadata preallocation.img
Formatting 'preallocation.img', fmt=qcow2 size=214748364800 encryption=off cluster_size=65536 preallocation='metadata' lazy_refcounts=off
# qemu-img create -f qcow2 -o size=200G non_preallocation.img
Formatting 'non_preallocation.img', fmt=qcow2 size=214748364800 encryption=off cluster_size=65536 lazy_refcounts=off
# ls -lh
-rw-r--r-- 1 root root 196K Jun 21 10:34 non_preallocation.img
-rw-r--r-- 1 root root 201G Jun 21 10:34 preallocation.img
# du -sh *
136K non_preallocation.img
32M preallocation.img
※ 로컬에서 복사(cp)를 하게될경우는 실제 데이타 싸이즈로 복사되지만 wget, ftp, scp 등등 다른곳으로 옮길때가 문제됨... 200G 그대로....
