Re: [PATCH 0/3] Implement support for QCOW2 data files
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.
Nikolai Barybin (3):
conf: add data-file feature and related fields to virStorageSource
storage: add qcow2 filename parsing from header
qemu: enable qcow2 data-file attach to VM on start
src/conf/storage_source_conf.c | 8 ++++
src/conf/storage_source_conf.h | 5 ++
src/qemu/qemu_block.c | 45 ++++++++++++++++++
src/qemu/qemu_cgroup.c | 3 ++
src/qemu/qemu_namespace.c | 6 +++
src/storage_file/storage_file_probe.c | 66 +++++++++++++++++++++++----
src/storage_file/storage_source.c | 38 ++++++++++++++-
A quick skim through the pathces shows following bits missing:
- any form of tests
- XML formating and parsing of the path (needed as you need the path of
the data file to be user-configurable for cases when user declares
the backing chain in the XML and also to be overridable. Additionally
we need to record the path for cases when libvirtd/virtqemud restarts
as we must not try to re-probe the image afterwards)
- security driver intergration (any of the above will not work if the
data file is not accessible as it won't be chown'd/relabelled)
Few more comments inline.