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[Qemu-devel] [Bug 660366] Re: "qemu-img convert -O qcow2 -o backing_file


From: sakishrist
Subject: [Qemu-devel] [Bug 660366] Re: "qemu-img convert -O qcow2 -o backing_file" makes huge images
Date: Mon, 10 Aug 2015 12:37:43 -0000

Is this reintroduced? I am on version 2.3.0

$ dd if=/dev/urandom of=disk bs=1M count=1024

$ qemu-img convert -f raw -O qcow2 disk disk.qcow

$ qemu-img convert -f raw -O qcow2 -o backing_file=disk.qcow disk
disk1.qcow

$ ls -l
total 3146388
-rw-r--r-- 1 sakis sakis 1073741824 10 авг 15,29 disk
-rw-r--r-- 1 sakis sakis 1074135040 10 авг 15,30 disk.qcow
-rw-r--r-- 1 sakis sakis 1074135040 10 авг 15,31 disk1.qcow

All the data is copied again.

$ qemu-img info disk1.qcow
image: disk1.qcow
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 1.0G
cluster_size: 65536
*backing file: disk.qcow*
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

Qemu-img works as expected though.

$ qemu-img create -f qcow2 -o backing_file=disk1.qcow disk2.qcow

$ ls -l
total 3146584
-rw-r--r-- 1 sakis sakis 1073741824 10 авг 15,29 disk
-rw-r--r-- 1 sakis sakis 1074135040 10 авг 15,30 disk.qcow
-rw-r--r-- 1 sakis sakis 1074135040 10 авг 15,31 disk1.qcow
-rw-r--r-- 1 sakis sakis     197120 10 авг 15,33 disk2.qcow

-- 
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https://bugs.launchpad.net/bugs/660366

Title:
  "qemu-img convert -O qcow2 -o backing_file" makes huge images

Status in QEMU:
  Fix Released

Bug description:
  $ dd if=/dev/urandom bs=1M of=1.img count=4
  4+0 records in
  4+0 records out
  4194304 bytes (4,2 MB) copied, 1,0413 s, 4,0 MB/s
  $ qemu-img create -f qcow2 -b 1.img 2.img
  Formatting '2.img', fmt=qcow2 size=4194304 backing_file='1.img' 
encryption=off cluster_size=0 
  $ qemu-img convert -O qcow2 -o backing_file=1.img 2.img 3.img
  $ du -h ?.img
  4,1M  1.img
  144K  2.img
  4,3M  3.img

  The conversion result is bigger then the source!

  It appears that "-o backing_file" is not applied to data (as
  expected). I.e. all data is put into the resulting image: both from
  source image and "backing" image.

  Expected behavior is to put only data that is not present in
  backing_file.

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