[Qemu-devel] qcow2: slow internal snapshot creation

[Dietmar Maurer]

qcow2 snapshot on newly created files are fast:

 

# qemu-img create -f qcow2 test.img 200G

# time qemu-img snapshot -c snap1 test.img

real       0m0.014s

 

but if metadata is allocated it gets very slow:

 

# qemu-img create  -f qcow2 -o "preallocation=metadata" test.img 200G

# time qemu-img snapshot -c snap1 test.img

real       1m20.399s

 

but reading the metadata is also fast:

 

# time qemu-img check test.img

real       0m0.371s

 

So why is creating a new snapshot that slow – any ideas?