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"qemu-img compare" is unnecessarily slow on empty raw files: ----- 8< ----- $ for fmt in raw qcow2; do \ for f in a b; do \ qemu-img create -f $fmt $f 1G \ done \ time qemu-img compare a b \ done Formatting 'a', fmt=raw size=1073741824 Formatting 'b', fmt=raw size=1073741824 Images are identical. real 0m6.424s user 0m0.177s sys 0m6.194s Formatting 'a', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 Formatting 'b', fmt=qcow2 size=1073741824 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 Images are identical. real 0m0.008s user 0m0.005s sys 0m0.003s ----- 8< ----- Ideally, because the new created raw files are empty: $ qemu-img info a image: a file format: raw virtual size: 5.0G (5368709120 bytes) disk size: 0 Images are identical. The efficiency of qemu-img compare operation should be close to qcow2 if not more.
Upstream commit: commit 25ad8e6e95770d4f01844dc2542d861e73b174b2 Author: Fam Zheng <famz> Date: Wed Jan 13 16:37:41 2016 +0800 qemu-img: Speed up comparing empty/zero images
This bug was accidentally moved from POST to MODIFIED via an error in automation, please see mmccune with any questions
Reproduce this bug with following version: qemu-img-rhev-2.3.0-31.el7.x86_64 Steps: # for a in raw qcow2; do for b in c d; do qemu-img create -f $a $b 5G; done; time qemu-img compare c d; done Formatting 'c', fmt=raw size=5368709120 Formatting 'd', fmt=raw size=5368709120 Images are identical. real 0m3.616s user 0m0.455s sys 0m3.179s Formatting 'c', fmt=qcow2 size=5368709120 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 Formatting 'd', fmt=qcow2 size=5368709120 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 Images are identical. real 0m0.014s user 0m0.008s sys 0m0.006s So this bug reproduced --------------------------------------------------------- Verified this bug with following version: qemu-img-rhev-2.6.0-1.el7.x86_64 Steps: # for a in raw qcow2; do for b in c d; do qemu-img create -f $a $b 5G; done; time qemu-img compare c d; done Formatting 'c', fmt=raw size=5368709120 Formatting 'd', fmt=raw size=5368709120 Images are identical. real 0m0.011s user 0m0.007s sys 0m0.004s Formatting 'c', fmt=qcow2 size=5368709120 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 Formatting 'd', fmt=qcow2 size=5368709120 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16 Images are identical. real 0m0.012s user 0m0.008s sys 0m0.004s according to this result. The bug is fixed.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://rhn.redhat.com/errata/RHBA-2016-2673.html