[ovirt-devel] Re: Disk sizes not updated on unmap/discard

> Kevin, is this the expected behavior or a bug in qemu? > > The disk I tested is a single qcow2 image without the backing file, so > theoretically qemu can deallocate all the discarded clusters. This is expected. Discard just frees the cluster whereever it is stored, but it doesn't compact the image, i.e. move data at higher offsets to lower offsets (which would be a rather expensive operation). If your storage supports thin provisioning/hole punching (the most common case of this is sparse files on a filesystem), then you can use the freed space for something else. If it doesn't, it's just marked free on the qcow2 level and future writes to the image will allocate the freed space first instead of growing the image, but you won't be able to use it for things outside of the image. In contrast, 'qemu-img convert' starts with an empty file and only writes what needs to be written, so it will result in a compacted image file that doesn't have holes and is as short as it can be.