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@dupond.be> --- 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?
+ 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'
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.
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