> On Mon, Mar 14, 2011 at 1:53 PM, Chunqiang Tang <ct...@us.ibm.com> wrote: > > Therefore, during normal execution of a > > VM, images with snapshots are as fast as images without snapshot. > > Hang on, an image with a snapshot still needs to do copy-on-write, > just like backing files. The cost of copy-on-write is reading data > from the backing file, whereas a non-CoW write doesn't need to do > that. > > So no, snapshots are not free during normal execution.
You are right. For any implementation of snapshot (internal or external), this CoW overhead is unavoidable. What I meant to say was that, other than this mandatory CoW overhead, FVD's internal snapshot does not incur any additional metadata update overhead (unlike that in QCOW2).
