On Tue, Sep 12, 2017 at 20:49:12 +0300, Khasan Khafizov wrote: > Suppose we have a disk with several external snapshots of it. This disk is > attached to some domain. I found that after destroying and starting the > domain the xml of attached disk doesn't contain all backingStore that it > has to. > > Before the restart the part of domain xml that relates to attached disk > looks like this: > > <disk type='file' device='disk'> > > <driver name='qemu' type='qcow2'/> > > <source file='...'/> > > <backingStore type='file' index='1'> > > <format type='qcow2'/> <!-- here is qcow2 --> > > <source file='...'/> > > <backingStore type='file' index='2'> > > <format type='qcow2'/> > > <source file='...'/> > > <backingStore type='file' index='3'> > > <format type='qcow2'/> > > <source file='...'/> > > <backingStore/> > > </backingStore> > > </backingStore> > > </backingStore> > > </disk> > > > And after the restart like this: > > <disk type='file' device='disk'> > > <driver name='qemu' type='qcow2'/> > > <source file='...'/> > > <backingStore type='file' index='1'> > > <format type='raw'/> <!-- here is raw --> > > <source file='...'/> > > <backingStore/> > > </backingStore> > > </disk> Did you create the snapshot by specifying --reuse-external and providing a pre-created qcow2 image? > > So, the difference in two xml not only in depth of backing stores but in > format type also. > The result of it is that you can't run blockcommit that will commit overlay > into base image if they don't have direct relations (if for example overlay > is child of child of base image). Virsh says it can't find base in chain ( > exception > <https://github.com/libvirt/libvirt/blob/cdecfbed027ab242467580a897a636be82d5d411/src/util/virstoragefile.c#L1667> > ). > Then I got that libvirt stops calling virStorageFileGetMetadataRecurse > <https://github.com/libvirt/libvirt/blob/ed8661a309c96e4a1573dc0ca2c76ea7ff19469a/src/storage/storage_source.c#L394> > function > after first call because it thinks that format of first backingStore of > disk is raw instead of qcow2. I solved it by adding to the qemu config > the following line: > > > allow_disk_format_probing = 1 This is insecure. > > But in qemu config it was marked as secure hole. Yep. > Idea of fix is that we can try assume that backing store of qcow is qcow > insted of raw, but I really don't know what to do to implement this idea if > it at all possible If you are pre-creating the images you have to specify -o backing_fmt=qcow2 when creating them. Otherwise libvirt will treat it as raw since probing is not allowed.
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