[Pkg-libvirt-maintainers] Bug#1121280: libvirt0: libvirt snapshot creation disables detect_zeroes

[zrm]

Package: libvirt0
Version: 11.3.0-3+deb13u1
Severity: normal
Tags: upstream
X-Debbugs-Cc: debbug at trustiosity.com

During snapshot creation in libvirt with qemu/kvm, the function
qemuDomainPrepareDiskSourceData() is called to copy properties into the
new top image for the snapshot. The function only copies the detect_zeroes property if the image is already the top image:

    /* transfer properties valid only for the top level image */
    if (src == disk->src || src == disk->src->dataFileStore)
        src->detect_zeroes = disk->detect_zeroes;

The new image is about to be the top image, but it isn't yet at the time of
this function call, with the result that detect_zeroes remains as its default (off) and creating a snapshot causes it to be disabled in the running VM:

# grep -A3 "device='disk'" trogdor.xml
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' discard='unmap' detect_zeroes='unmap'/>
      <source file='/var/lib/libvirt/images/trogdor.qcow2'/>
      <target dev='vda' bus='virtio'/>
# virsh define trogdor.xml
Domain 'trogdor' defined from trogdor.xml
# virsh start trogdor
Domain 'trogdor' started

# virsh qemu-monitor-command trogdor --pretty
'{"execute":"query-block"}'|grep detect_zeroes
        "detect_zeroes": "unmap",

# virsh snapshot-create-as --domain trogdor snap42 --diskspec
vda,file=/var/lib/libvirt/images/trogdor-snap42.qcow2 --disk-only

Domain snapshot snap42 created

# virsh qemu-monitor-command trogdor --pretty
'{"execute":"query-block"}'|grep detect_zeroes
        "detect_zeroes": "off",


-- System Information:
Debian Release: 13.2
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 6.12.57+deb13-amd64 (SMP w/4 CPU threads; PREEMPT)
Kernel taint flags: TAINT_FIRMWARE_WORKAROUND
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages libvirt0 depends on:
ii  libacl1             2.3.2-2+b1
ii  libapparmor1        4.1.0-1
ii  libaudit1           1:4.0.2-2+b2
ii  libbsd0             0.12.2-2
ii  libc6               2.41-12
ii  libcap-ng0          0.8.5-4+b1
ii  libcurl3t64-gnutls  8.14.1-2+deb13u2
ii  libgcc-s1           14.2.0-19
ii  libglib2.0-0t64     2.84.4-3~deb13u1
ii  libgnutls30t64      3.8.9-3
ii  libjson-c5          0.18+ds-1
ii  libnl-3-200         3.7.0-2
ii  libnuma1            2.0.19-1
ii  libsasl2-2          2.1.28+dfsg1-9
ii  libselinux1         3.8.1-1
ii  libssh-4            0.11.2-1+deb13u1
ii  libssh2-1t64        1.11.1-1
ii  libtirpc3t64        1.3.6+ds-1
ii  libvirt-common      11.3.0-3+deb13u1
ii  libxml2             2.12.7+dfsg+really2.9.14-2.1+deb13u2

Versions of packages libvirt0 recommends:
ii  libvirt-l10n  11.3.0-3+deb13u1

libvirt0 suggests no packages.

-- no debconf information