Re: [PATCH 3/5] conf: Introduce <metadata_cache> subelement of <disk><driver>

In certain specific cases it might be beneficial to be able to control the metadata caching of storage image format drivers of a hypervisor. Introduce XML machinery to set the maximum size of the metadata cache which will be used by qemu's qcow2 driver. Signed-off-by: Peter Krempa <pkrempa(a)redhat.com&gt; --- docs/formatdomain.rst | 43 ++++++++++++++ docs/schemas/domaincommon.rng | 20 ++++++- src/conf/domain_conf.c | 51 ++++++++++++++-- src/util/virstoragefile.c | 1 + src/util/virstoragefile.h | 2 + .../qemuxml2argvdata/disk-metadata-cache.xml | 46 +++++++++++++++ .../disk-metadata-cache.x86_64-latest.xml | 58 +++++++++++++++++++ tests/qemuxml2xmltest.c | 1 + 8 files changed, 216 insertions(+), 6 deletions(-) create mode 100644 tests/qemuxml2argvdata/disk-metadata-cache.xml create mode 100644 tests/qemuxml2xmloutdata/disk-metadata-cache.x86_64-latest.xml diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst index 1189795974..abf85064fa 100644 --- a/docs/formatdomain.rst +++ b/docs/formatdomain.rst @@ -2735,6 +2735,11 @@ paravirtualized driver is specified via the ``disk`` element. ``format`` The ``format`` element contains ``type`` attribute which specifies the internal format of the backing store, such as ``raw`` or ``qcow2``. + + The ``format`` element can contain ``metadata_cache`` subelement, which + has identical semantics to the identically named subelement of ``driver`` + of a ``disk``. + ``source`` This element has the same structure as the ``source`` element in ``disk``. It specifies which file, device, or network location contains the data of @@ -2947,6 +2952,44 @@ paravirtualized driver is specified via the ``disk`` element. virtio-blk. ( :since:`Since 3.9.0` ) - For virtio disks, `Virtio-specific options <#elementsVirtio>`__ can also be set. ( :since:`Since 3.5.0` ) + - The optional ``metadata_cache`` subelement controls aspects related to the + format specific caching of storage image metadata. Note that this setting + applies only on the top level image; the identically named sublelement of + ``backingStore``'s ``format`` element can be used to specify cache + settings for the backing image. + + :since:`Since 7.0.0` the maximum size of the metadata cache of ``qcow2`` + format driver of the ``qemu`` hypervisor can be controlled via the + ``max_size`` subelement (see example below). + + In the majority of cases the default configuration used by the hypervisor + is sufficient so modifying this setting should not be necessary. For + specifics on how the metadata cache of ``qcow2`` in ``qemu`` behaves refer + to the ``qemu`` + `qcow2 cache docs <https://git.qemu.org/?p=qemu.git;a=blob;f=docs/qcow2-cache.txt>`__ + + **Example:** + +:: + + <disk type='file' device='disk'> + <driver name='qemu' type='qcow2'> + <metadata_cache> + <max_size unit='bytes'>1234</max_size> + </metadata_cache> + </driver> + <source file='/var/lib/libvirt/images/domain.qcow'/> + <backingStore type='file'> + <format type='qcow2'> + <metadata_cache> + <max_size unit='bytes'>1234</max_size> + </metadata_cache>

+ </format> + <source file='/var/lib/libvirt/images/snapshot.qcow'/> + <backingStore/> + </backingStore> + <target dev='vdd' bus='virtio'/> + </disk> ``backenddomain`` The optional ``backenddomain`` element allows specifying a backend domain