[Qemu-devel] [Bug 322602] Re: Snapshot usage makes qcow2 image unusable due to large tables

[Jes Sorensen]

Hi,

Could you please let us know whether this is still a problem and if it
isn't, lets close this bug.

 In addition, you haven't listed which version of qemu-kvm you are
running. I am guessing it is something Ubuntu based based on the version
number, but since not all of us are running Ubuntu it would be useful to
have more details.

Thanks,
Jes

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Snapshot usage makes qcow2 image unusable due to large tables
https://bugs.launchpad.net/bugs/322602
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Status in QEMU: New

Bug description:
To reproduce with 0.9.1 and svn:
- Create a 20G (or some size much greater than system RAM) qcow2 image
- Inside VM, install some OS, formatting whole drive
- Create snapshot with savevm
- Inside VM, reformat and reinstall OS
- Create snapshot with savevm
[...]

Eventually, qemu crashes, then neither qemu-img nor qemu can open the image 
because memory is exhausted.  The reason is that the whole refcount_table is 
loaded into memory, and this refcount_table has now become much bigger than the 
size of memory due to each snapshot taken after significant changes to the disk 
image.

The refcount_table really needs to be loaded and used in fixed size chunks to 
avoid this problem.  It will only get worse as typical storage set 
modifications during work sessions between snapshots outpace the size of system 
RAM.

Alternatively, there needs to be a way to "rollback" a snapshot without loading 
the whole disk image normally, so that a snapshot which has made the image 
unusable in this way can be reversed.