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I'm having difficulties to understand the disk size of my qcow2 image. I have a CentOS 6 box running:

# virsh version
Compiled against library: libvirt 0.10.2
Using library: libvirt 0.10.2
Using API: QEMU 0.10.2
Running hypervisor: QEMU 0.12.1

I run couple guests there and without much activity on the guests I noticed the backup ( I do manual complete file copy with cp, no qcow2 based snaps) on one of my guests has grown 4 times. The other guests seem to behave normally and have normal backup size growth.

When I login to that guest I see that 

# df -h
Filesystem                    Size  Used Avail Use% Mounted on
udev                          2.0G     0  2.0G   0% /dev
tmpfs                         396M  5.5M  391M   2% /run
/dev/mapper/debian9--vg-root  188G  2.7G  176G   2% /
tmpfs                         2.0G     0  2.0G   0% /dev/shm
tmpfs                         5.0M  4.0K  5.0M   1% /run/lock
tmpfs                         2.0G     0  2.0G   0% /sys/fs/cgroup
/dev/vda1                     236M   62M  162M  28% /boot
tmpfs                          89M     0   89M   0% /run/user/0

but the qcow2 file has grown from 5GB to 

# du -h /backups/vm01/20180111/vm01.qcow2 
19G /backups/vm01/20180111/vm01.qcow2

I found the size of qcow2 disk file grows rapidly and tried to "qemu-img convert" the backup file, but did not solve the problem. When I did dd if=/dev/zero of=vm01.qcow2 it ran until I ran out of space on that volume group ( more than the 19G ). I was expecting the qcow2 file to grow more or less with the size of the internal file system. Any hints what I may be doing wrong?

Regards, Pavel

ppavlov
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    Since this question isn't really about programming, it doesn't seem to be a good fit for [SO]. Maybe [SU] or [SF] might be more appropriate. – Robᵩ Jan 11 '18 at 22:39

1 Answers1

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Unless you have TRIM/DISCARD enabled for the host filesystem, QEMU and the guest OS, the qcow2 file will never shrink in size. So most likely explanation is that something in the guest OS created a very large file for a short time and then deleted it again. the qcow2 image would have grown to hold this file, but once the file was deleted, the qcow2 image won't shrink again, without TRIM/DISCARD being available.

DanielB
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