On a Thursday in 2021, Peter Krempa wrote:
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@xxxxxxxxxx>
---
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>
I don't like introducing a snake_case element under a camelCase one. But regardless of that, our XML is a mix of both of those and skewer-case. See also: https://www.redhat.com/archives/libvir-list/2021-January/msg00371.html Jano
+ </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
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