Re: [libvirt-users] Problem about Disk Dize of Destination and Source File after Do Blockcopy

On 07/21/2014 10:01 PM, shyu wrote: > # rpm -q libvirt qemu-kvm-rhev > libvirt-1.1.1-29.el7.x86_64 > qemu-kvm-rhev-1.5.3-60.el7ev_0.2.x86_64 These are downstream builds. Can you reproduce your situation with upstream libvirt 1.2.6 and qemu 2.1-rc2? It may be that you are hitting behavior that was introduced by downstream backports.

> 1. Check source file > # qemu-img info /var/lib/libvirt/images/rhel6.img > image: /var/lib/libvirt/images/rhel6.img > file format: qcow2 > virtual size: 5.0G (5368709120 bytes) > disk size: 1.2G Disk size tracks how much of the qcow2 file has been allocated, NOT how much guest data has been allocated. > 3. Check destination file's disk size > > # qemu-img info /var/lib/libvirt/images/copy.img > image: /var/lib/libvirt/images/copy.img > file format: qcow2 > virtual size: 5.0G (5368709120 bytes) > disk size: 2.0G

The thing to remember here is that blockcopy defaults to doing a cluster at a time, even if the guest has not yet touched every sector within the cluster. It may be that you are hitting cases where the copy operation ends up writing an entire cluster in the destination where only a partial cluster had been allocated in the source. But that does not necessarily mean the copy is flawed, only that the default granularity was large enough to inflate the destination with redundant all-zero sectors in the interest of speeding up the operation, or that the destination is not as sparse as the source. Qemu offers the 'granularity' parameter to the 'drive-mirror' command to alter the granularity, but libvirt is not (currently) exposing this knob to the user so for now libvirt is just relying on qemu defaults. It may also be a factor of how much copy-on-write dirtying is happening. If the guest is actively hammering on the disk during the copy operation, the same cluster may be marked dirty multiple times; if qemu allocates a new destination cluster for each pass through the dirty bitmap, it may result in some inflation in size due to clusters that are written early then discarded as they are later rewritten in a new allocation. I'm not familiar enough with qemu block handling to know if this is happening, or even whether qemu could be patched to do better garbage collection of clusters left unused if it is happening. There is nothing that libvirt can do about this. I don't think it is a bug, but you may want to ask on the qemu list, since it is up to qemu whether or not the copy will be inflated in host size. But inflation is not a bad thing in itself - the real question is whether the copy contains the same guest contents as the original at the time the copy completed - as long as that is the case, even if the host sizes are different, then the copy is reliable.