升级有风险，建议先备份数据。
¶ 8升级到8-stream
USTC mirror将CentOS 8升级为stream
¶ 8-stream升级到9-stream
先将原先的repos备份：
mv /etc/yum.repos.d /etc/yum.repos.d-$(date +%Y.%m.%d.%H:%M:%S.%N)然后再指定新的源。以TUNA镜像源为例：
mkdir /etc/yum.repos.d
# tuna
mirror=https://mirrors.tuna.tsinghua.edu.cn/centos-stream
# aliyun
# mirror=https://mirrors.aliyun.com/centos-stream
cat > /etc/yum.repos.d/centos.repo <<EOF
[baseos]
name=CentOS Stream \$releasever - BaseOS
baseurl=$mirror/\$stream/BaseOS/\$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
[appstream]
name=CentOS Stream \$releasever - AppStream
baseurl=$mirror/\$stream/AppStream/\$basearch/os/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
EOF
cat > /etc/yum.repos.d/centos-addons.repo <<EOF
[extras-common]
name=CentOS Stream \$releasever - Extras packages
baseurl=$mirror/SIGs/\$stream/extras/\$basearch/extras-common/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-SIG-Extras-SHA512
EOF然后将$releasever设置为9，将$stream设置为9-stream:
# https://donghao.org/2015/04/30/the-value-of-releasever-for-yum/
echo 9 > /etc/yum/vars/releasever
echo 9-stream > /etc/yum/vars/streamyum makecache这时可能会报这种错：nothing provides module(platform:el8) needed by module virt:rhel:8080020230131213515:fd72936b.x86_64。这些module都是过时的，我们直接reset这些module即可:
# https://ahelpme.com/linux/centos-stream-9/dnf-install-and-conflicting-requests-nothing-provides-moduleplatformel8-needed-by-module/
yum module reset virt然后升级：
# https://dnf.readthedocs.io/en/latest/command_ref.html#distro-sync-command
# https://dnf.readthedocs.io/en/latest/conf_ref.html
# --allowerasing: 允许删除在新版本repo中不存在的包
# deltarpm=false: 关闭差量传输
sudo yum distro-sync --allowerasing --setopt=deltarpm=false
# https://bugzilla.redhat.com/show_bug.cgi?id=1960991
rpmdb --rebuilddb注意，CentOS 9 stream创建的XFS分区会引入与旧内核不兼容的新特性：https://bugzilla.redhat.com/show_bug.cgi?id=2046431。不过从CentOS 8 stream升级到CentOS 9 stream不会导致在已有的分区中引入该新特性。
¶ epel
# /etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-9
yum install epel-release
# tuna
mirror=https://mirrors.tuna.tsinghua.edu.cn/epel
# aliyun
# mirror=https://mirrors.aliyun.com/epel
cat > /etc/yum.repos.d/epel.repo <<EOF
[epel]
name=Extra Packages for Enterprise Linux \$releasever - \$basearch
baseurl=$mirror/\$releasever/Everything/\$basearch/
enabled=1
gpgcheck=1
countme=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-\$releasever
[epel-debuginfo]
name=Extra Packages for Enterprise Linux \$releasever - \$basearch - Debug
baseurl=$mirror/\$releasever/Everything/\$basearch/debug/
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-\$releasever
gpgcheck=1
[epel-source]
name=Extra Packages for Enterprise Linux \$releasever - \$basearch - Source
baseurl=$mirror/\$releasever/Everything/source/tree/
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-\$releasever
gpgcheck=1
EOF
cat > /etc/yum.repos.d/epel-next.repo <<EOF
[epel-next]
name=Extra Packages for Enterprise Linux \$releasever - Next - \$basearch
baseurl=$mirror/next/\$releasever/Everything/\$basearch/
enabled=1
gpgcheck=1
countme=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-\$releasever
[epel-next-debuginfo]
name=Extra Packages for Enterprise Linux \$releasever - Next - \$basearch - Debug
baseurl=$mirror/next/\$releasever/Everything/\$basearch/debug/
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-\$releasever
gpgcheck=1
[epel-next-source]
name=Extra Packages for Enterprise Linux \$releasever - Next - \$basearch - Source
baseurl=$mirror/next/\$releasever/Everything/source/tree/
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-\$releasever
gpgcheck=1
EOF
yum update --allowerasing¶ 事务测试失败
file /usr/share/ruby/irb from install of rubygem-irb-1.3.5-160.el9.noarch conflicts with file from package ruby-irb-2.5.9-107.module_el8.4.0+847+ee687b6c.noarch好像sudo yum remove ruby-irb就行了。
相关：https://forums.centos.org/viewtopic.php?t=50923
¶ grub2-editenv：错误： 无效的环境块
# https://gist.github.com/illucent/c3be9da2b87d79a22112436b93642c9b
# https://www.cnblogs.com/5201351/p/17134924.html
grub2-editenv /boot/grub2/grubenv create