performance is also reduced when snapshot exist. (like if they are no preallocated metadatas)
see initial git commit http://git.qemu.org/?p=qemu.git;a=commit;h=a35e1c177debb01240243bd656caca302410d38c "qcow2: Metadata preallocation This introduces a qemu-img create option for qcow2 which allows the metadata to be preallocated, i.e. clusters are reserved in the refcount table and L1/L2 tables, but no data is written to them. Metadata is quite small, so this happens in almost no time. Especially with qcow2 on virtio this helps to gain a bit of performance during the initial writes. However, as soon as create a snapshot, we're back to the normal slow speed, obviously. So this isn't the real fix, but kind of a cheat while we're still having trouble with qcow2 on virtio." ----- Mail original ----- De: "Dietmar Maurer" <[email protected]> À: [email protected] Cc: "Kevin Wolf ([email protected])" <[email protected]> Envoyé: Vendredi 23 Novembre 2012 08:26:13 Objet: [Qemu-devel] qcow2: slow internal snapshot creation 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?
