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

[Ján Tomko]

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 at redhat.com>
>---
> 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

Also:
s/lelement/element/

Jano

>+      ``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>`__
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