On Mon, Aug 12, 2024 at 09:26:08 +0200, Peter Krempa wrote: > On Sun, Aug 11, 2024 at 17:34:45 +0300, Nikolai Barybin via Devel wrote: > > There are use cases when the existing disks (i.e. LVM) are wanted > > to be used with advanced features. For this purpose QEMU allows > > data-file feature for qcow2 files: metadata is kept in the qcow2 > > file like usual, but guest data is written to an external file. > > These patches enable support for this feature in libvirt. > > So this feature was once attempted to be added but was never finished > and the comitted bits were reverted eventually. (I've purged my local > archive so I don't have the link handy but I can look if you want the > links to the old posting) > > It was deemed that this doesn't really add any performance benefit over > storing the actual qcow2 with data inside. The qcow2 with data can be > stored inside the LV or other block device for that matter and thus can > provide all features that are necessary. The data file feature makes > also the management of the metadata and data much more complex, for a > very bad trade-off. At this point with 'qemu-img measure' it's easy to > query the necessary size to have a fully allocated qcow2 inside the > block device. > > Based on the history of this I'd like to ask you to summarize > justifications and reasons for adding this feature before continuing. Based on the current state of the series and what would be required to make it viable to be accepted I very strongly suggest re-thinking if you really need this feature, especially based on the caveats above.
