Re: [Libguestfs] question about backing file path of a qcow2 image

On Wed, Sep 11, 2013 at 10:33:06AM +0800, bigclouds wrote: > hi,all > images of qcow2 format with backing files, its backing files have > two path infos (absolute path and relative path), This is not true. The file format only contains the relative path. qemu-img info will print an absolute path, but it's not stored inside the file. > how to let > libguestfs use absolute path rather than relative path. in some > case, this leads error. It's hard to know what the problem is you're describing since you give no details at all. But anyway ... Libguestfs doesn't actually use or care about the backing file of qcow2 files. That is entirely handled by qemu when it opens the qcow2 file. There are three problems I know about: (1) If your backing file is located on tmpfs, then qemu can have problems opening the file. This is fixed upstream (commit 749e947bb010 / RHBZ#994517). (2) If your disk image refers to the backing file by its relative path then it may not work if you move it around. This should be obvious, but seems to cause people enough trouble. (3) Certain characters in filenames may be forbidden (esp. ':') or may require quoting (','). Libguestfs (recent versions, anyhow) should always do the right thing when it encounters one of these characters. If not, file a bug. The solution to (2) is to always use absolute paths to files and/or not to move files around. So long as you don't move the file around, qemu handles the case where it is started from a different directory. It searches for the backing file relative to the location of the qcow2 file, NOT relative to the current directory. If you find some other problem, please show us a clear reproducer and include all of the debugging and error messages: http://libguestfs.org/guestfs-faq.1.html#debug Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org