On 8/06/2023 16:36, Peter Krempa wrote:
On Tue, Jun 06, 2023 at 11:54:40 +0200, Jean-Louis Dupond wrote:
Qemu 8.1.0 will add discard_no_unref option for qcow2 images.
When this option is enabled (default=false), then it will no longer
unreference clusters when guest does a discard, but it will just free
the blocks (useful for incremental backups for example) and pass the
discard to the lower layer.
This was implemented to avoid fragmentation within the qcow2 image.
Signed-off-by: Jean-Louis Dupond <jean-louis@xxxxxxxxx>
---
docs/formatdomain.rst | 6 +++
src/conf/domain_conf.c | 8 +++
src/conf/domain_conf.h | 1 +
src/conf/domain_validate.c | 15 ++++++
src/conf/schemas/domaincommon.rng | 8 +++
src/conf/storage_source_conf.c | 1 +
src/conf/storage_source_conf.h | 1 +
src/qemu/qemu_block.c | 11 ++---
src/qemu/qemu_domain.c | 1 +
src/qemu/qemu_driver.c | 4 +-
src/vz/vz_utils.c | 6 +++
tests/qemusecuritytest.c | 1 +
.../disk-discard_no_unref.x86_64-latest.args | 39 +++++++++++++++
.../disk-discard_no_unref.xml | 39 +++++++++++++++
tests/qemuxml2argvtest.c | 1 +
.../disk-discard_no_unref.x86_64-latest.xml | 49 +++++++++++++++++++
tests/qemuxml2xmltest.c | 1 +
17 files changed, 185 insertions(+), 7 deletions(-)
create mode 100644 tests/qemuxml2argvdata/disk-discard_no_unref.x86_64-latest.args
create mode 100644 tests/qemuxml2argvdata/disk-discard_no_unref.xml
create mode 100644 tests/qemuxml2xmloutdata/disk-discard_no_unref.x86_64-latest.xml
diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
index c3526439bf..6e89f8e01f 100644
--- a/docs/formatdomain.rst
+++ b/docs/formatdomain.rst
@@ -3284,6 +3284,12 @@ paravirtualized driver is specified via the ``disk`` element.
format driver of the ``qemu`` hypervisor can be controlled via the
``max_size`` subelement (see example below).
+ The optional ``discard_no_unref`` attribute can be set to control the way
+ the ``qemu`` hypervisor handles guest discard commands inside the qcow2
+ image. When enabled, a discard from within the guest will not cause the
+ qcow2 image to remove the cluster references, but will still send the
+ discard command to its lower layer. :since:`Since 9.5.0`
I'm not quite sure that I understand from this description what it
actually does.
I understand that if a discard request from the guest gets to qemu, the
QCOW2 image keeps the reference, but the backing image unreferences it?
What would this be useful for?
When a guest does a discard request, it will normally unreference the
discarded block in the qcow2 image.
Now this creates a lot of fragmentation in the image, and next to that
causes the image to grow sometimes over 110%
of its virtual size. Because when allocating a new block, it always
needs a continuous block, and because of the
default behavior, there are a lot of small holes in the qcow2 image
which doesn't fit the new allocations.
But if we enable this option, then the clusters will not be
unreferenced, which will fix the fragmentation,
as there will be no holes anymore in the qcow2 image.
Some more info in https://gitlab.com/qemu-project/qemu/-/issues/1621
+
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
[...]
diff --git a/src/conf/domain_validate.c b/src/conf/domain_validate.c
index 80d6a2ffd9..34508b1474 100644
--- a/src/conf/domain_validate.c
+++ b/src/conf/domain_validate.c
@@ -379,6 +379,12 @@ virDomainDiskVhostUserValidate(const virDomainDiskDef *disk)
return -1;
}
+ if (disk->discard_no_unref) {
+ virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+ _("discard_no_unref is not supported with vhostuser disk"));
+ return -1;
+ }
[1]
+
/* Unsupported driver elements */
if (disk->src->metadataCacheMaxSize > 0) {
@@ -921,6 +927,15 @@ virDomainDiskDefValidate(const virDomainDef *def,
return -1;
}
+ if (disk->discard_no_unref == VIR_TRISTATE_SWITCH_ON) {
+ if (disk->src->readonly) {
+ virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+ _("discard_no_unref is not compatible with read-only disk '%1$s'"),
+ disk->dst);
+ return -1;
+ }
+ }
+
Validation that the flag is used with qcow2 is missing. If you add that
[1] becomes pointless as vhost-user doesn't work with 'qcow2'
Fixed
diff --git a/src/conf/schemas/domaincommon.rng b/src/conf/schemas/domaincommon.rng
index c1725bb511..0b49fbcfeb 100644
--- a/src/conf/schemas/domaincommon.rng
+++ b/src/conf/schemas/domaincommon.rng
@@ -2514,6 +2514,9 @@
<optional>
<ref name="detect_zeroes"/>
</optional>
+ <optional>
+ <ref name="discard_no_unref"/>
+ </optional>
You can declare the attribute directly here, having a definition for it
doesn't make too much sense.
As the others were all with a reference, I did it also for this one. But
fixed.
diff --git a/tests/qemuxml2argvdata/disk-discard_no_unref.xml b/tests/qemuxml2argvdata/disk-discard_no_unref.xml
new file mode 100644
index 0000000000..34d61b9879
--- /dev/null
+++ b/tests/qemuxml2argvdata/disk-discard_no_unref.xml
@@ -0,0 +1,39 @@
+<domain type='qemu'>
+ <name>test</name>
+ <uuid>92d7a226-cfae-425b-a6d3-00bbf9ec5c9e</uuid>
+ <memory unit='KiB'>1048576</memory>
+ <currentMemory unit='KiB'>1048576</currentMemory>
+ <vcpu placement='static'>1</vcpu>
+ <os>
+ <type arch='x86_64' machine='pc'>hvm</type>
+ <boot dev='cdrom'/>
+ <boot dev='hd'/>
+ <bootmenu enable='yes'/>
+ </os>
+ <clock offset='utc'/>
+ <on_poweroff>destroy</on_poweroff>
+ <on_reboot>restart</on_reboot>
+ <on_crash>restart</on_crash>
+ <devices>
+ <emulator>/usr/bin/qemu-system-x86_64</emulator>
+ <!-- For this disk, intentionally stress parser resilience to
+ atypical ordering -->
+ <disk device='disk'>
+ <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
+ <source file='/var/lib/libvirt/images/f14.img'/>
+ <target dev='vda' bus='virtio'/>
+ <driver discard='unmap' discard_no_unref='on' name='qemu' type='qcow2'/>
+ </disk>
+ <disk type='file' device='cdrom'>
+ <driver discard='ignore'/>
+ <source file='/var/lib/libvirt/Fedora-14-x86_64-Live-KDE.iso'/>
+ <target dev='hdc' bus='ide'/>
+ <readonly/>
+ <address type='drive' controller='0' bus='1' target='0' unit='0'/>
+ </disk>
Please drop this irrelevant disk
Fixed
I'll post a v2 version asap.
Thanks
Jean-Louis
