Preparing a Linux Image

Last updated: 2021-04-06 11:48:50

    Overview

    This document describes how to create a system disk image of your local Linux server or Linux server deployed on other cloud platforms.

    Directions

    Preparations

    Before preparing and exporting a system disk image, complete the following checks.

    Note:

    If you need to prepare and export a data disk image, skip this operation.

    Checking the partitioning and start mode of the OS

    1. Run the following command to check whether the OS partition is a GPT partition.
      sudo parted -l /dev/sda | grep 'Partition Table'
      
    • If the returned result is msdos, the MBR partition is used. In this case, proceed to the next step.
    • If the returned result is gpt, the GPT partition is used, which is currently unavailable for the service migration. In this case, submit a ticket.
    1. Run the following command to check whether the OS starts in EFI mode.
      sudo ls /sys/firmware/efi
      
    • If the EFI file exists, the operating system starts in EFI mode. In this case, submit a ticket.
    • If no file exists, proceed to the next step.

    Checking system-critical files

    Check system-critical files including but not limited to the following:

    Note:

    Follow the distribution standards to ensure that the paths and permissions of the system-critical files are correct and the files can be read and written normally.

    • /etc/grub2.cfg: it’s recommended to use uuid in the kernel parameter for mounting root. Other methods (such as root=/dev/sda) may cause a system startup failure.
    • /etc/fstab: no other disks are mounted. Otherwise, these disks may be lost and cause the system startup failure after migration.
    • /etc/shadow: granted with the read-write permissions.

    Uninstalling software

    Uninstall the conflicting drivers and software programs (including VMware tools, Xen tools, Virtualbox GuestAdditions, and other software that comes with underlying drivers).

    Checking the virtio driver

    For more information, see Checking Virtio Drivers in Linux.

    Installing cloud-init

    For more information, see Installing Cloud-Init on Linux.

    Checking other hardware configurations

    After the migration to the cloud, hardware changes include but are not limited to:

    • The graphics card changes to Cirrus VGA.
    • The disk changes to Virtio Disk. The device name is vda or vdb.
    • The ENI changes to Virtio Nic. By default, only eth0 is available.

    Querying partitions and their sizes

    Run the following command to query the current OS partition format and determine the partitions to be copied and their sizes.

    mount
    

    A result similar to the following is returned:

    proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
    sys on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
    dev on /dev type devtmpfs (rw,nosuid,relatime,size=4080220k,nr_inodes=1020055,mode=755)
    run on /run type tmpfs (rw,nosuid,nodev,relatime,mode=755)
    /dev/sda1 on / type ext4 (rw,relatime,data=ordered)
    securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
    tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
    devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
    tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
    cgroup on /sys/fs/cgroup/unified type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
    cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,name=systemd)
    pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
    cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
    cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
    cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,rdma)
    cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
    cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
    cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
    cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
    cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
    cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
    cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
    cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
    systemd-1 on /home/libin/work_doc type autofs (rw,relatime,fd=33,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=12692)
    systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=39,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=12709)
    debugfs on /sys/kernel/debug type debugfs (rw,relatime)
    mqueue on /dev/mqueue type mqueue (rw,relatime)
    hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,pagesize=2M)
    tmpfs on /tmp type tmpfs (rw,nosuid,nodev)
    configfs on /sys/kernel/config type configfs (rw,relatime)
    tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=817176k,mode=700,uid=1000,gid=100)
    gvfsd-fuse on /run/user/1000/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=1000,group_id=100)
    

    According to the result, the root partition resides in /dev/sda1. No independent partitions reside in /boot or /home. sda1 contains the boot partition, and mbr is missing. Therefore, we only need to copy the entire sda.

    Note:

    The exported image should contain at least the root partition and mbr. If mbr is missing, the operating system cannot be started.
    If /boot and /home are independent partitions in the current operating system, the exported image should also contain them.

    Exporting an image

    Choose the appropriate image export method as needed.

    Using a platform tool to export an image

    For more information on how to use the image export tools of virtualization platforms, such as VMWare vCenter Convert and Citrix XenConvert, see the document for the respective platform.

    Note:

    Tencent Cloud’s service migration supports images in qcow2, vhd, raw, and vmdk formats.

    Using commands to export an image

    Note:

    This method poses higher risks. For example, the file system's metadata may be corrupted when I/O is busy. We recommended that you check the image to make sure that the image is intact and correct after it is exported.

    You can use either the qemu-img or dd command to export an image.

    • Use the qemu-img command
    1. Run the following command to install the required package. This document uses Debian as an example. The package name may vary by distributions, such as qemu-img for CentOS.
      apt-get install qemu-utils
      
    2. Run the following command to export /dev/sda to /mnt/sdb/test.qcow2.
      sudo qemu-img convert -f raw -O qcow2 /dev/sda /mnt/sdb/test.qcow2
      
      In this command, /mnt/sdb indicates the mounted new disk or another network storage.
      To convert its format, modify the value of the -O parameter to one of the following:
      Parameter ValueDescription
      qcow2qcow2 format
      vpcvhd format
      vmdkvmdk format
      rawNo format
    • Use the dd command
      For example, run the following command to export an image in raw format.

      sudo dd if=/dev/sda of=/mnt/sdb/test.imag bs=1K count=$count
      

      The count parameter specifies the number of partitions to be copied, which can be queried by running the fdisk command. To copy all partitions, ignore count.
      For example, run the following command to view the number of partitions of /dev/sda.

      fdisk -lu /dev/sda
      

      A result similar to the following is returned:

      Disk /dev/sda: 1495.0 GB, 1494996746240 bytes
      255 heads, 63 sectors/track, 181756 cylinders, total 2919915520 sectors
      Units = sectors of 1 * 512 = 512 bytes
      Sector size (logical/physical): 512 bytes / 4096 bytes
      I/O size (minimum/optimal): 4096 bytes / 4096 bytes
      Disk identifier: 0x0008f290
      
       Device Boot      Start         End      Blocks   Id  System
      /dev/sda1   *        2048    41945087    20971520   83  Linux
      /dev/sda2        41945088    46123007     2088960   82  Linux swap / Solaris
      /dev/sda3        46123008    88066047    20971520   83  Linux
      /dev/sda4        88066048  2919910139  1415922046   8e  Linux LVM
      

      According to the returned result of the fdisk command, the sda1 ends at 41945087 * 512 bytes, so set count to 20481 MB.

      Note:

      The image exported by using the dd command is in raw format. We recommend that you convert it to qcow2, vhd, or other image formats.

    Converting the image format

    Note:

    Currently, Tencent Cloud’s service migration supports images in qcow2, vpc, vmdk, and raw formats. We recommend using the compressed image format to shorten the transmission and migration time.

    Convert the image format using the qemu-img command.
    For example, run the following command to convert the image in raw format to the qcow2 format.

    sudo qemu-img convert -f raw -O qcow2 test.img test.qcow2
    
    • -f is the source image file format.
    • -O indicates the destination image format. For the supported formats, see -O Parameter Values.

    Checking the image

    Note:

    The image file system that you prepare may be corrupted because you prepared the image without stopping the service or due to other reasons. Therefore, we recommend that you check the image after preparing it.

    If the image format is supported by the current platform, you can directly open and check the image file system. For example, the Windows platform supports VHD images, the Linux platform allows you to use qemu-nbd to open QCOW2 images, and the Xen platform allows you to directly open VHD files. This document uses the Linux platform as an example:

    1. Run the following commands in sequence to check whether the nbd component exists.
      modprobe nbd
      
      lsmod | grep nbd
      
      If a result similar to the following is returned, the nbd component exists. If nothing is returned, check whether the kernel compilation option CONFIG_BLK_DEV_NBD is enabled. If not, enable it or change the system before compiling the kernel again.
    2. Run the following commands in sequence to check the image.
      qemu-nbd -c /dev/nbd0 xxxx.qcow2
      
      mount /dev/nbd0p1 /mnt
      
      After you run the qemu-nbd command, /dev/nbd0 maps to xxx.qcow2, and /dev/nbd0p1 indicates the first partition of the virtual disk. If nbd0p1 does not exist or mount fails, the image may be incorrect.
      You can also start the CVM to check whether the image file works before uploading the image.