Linux Image Creation

Last updated: 2020-05-08 12:35:18


    This document describes how to create a Linux image.



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

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

    Checking the partitioning mode and launch mode of the operating system

    1. Run the following command to check whether the operating system partition is a GPT partition.
      sudo parted -l /dev/sda | grep 'Partition Table'
      • If the returned result is msdos, the partition is an MBR partition. In this case, go to the next step.
      • If the returned result is gpt, the partition is a GPT partition. Currently, service migration does not support GPT partitions. In this case, submit a ticket.
    2. Run the following command to check whether the operating system launch mode is EFI.
      sudo ls /sys/firmware/efi
      • If the EFI file exists, the current launch mode is EFI. In this case, submit a ticket.
      • If the EFI file does not exist, proceed to the next step.

    Checking system-critical files

    System-critical files to be checked include but are not limited to:

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

    • /etc/grub/grub.cfg: in the kernel parameter, uuid is recommended for mounting root. Other methods (such as root=/dev/sda) may cause failure in launching the system.
    • /etc/fstab: do not mount other disks. After migration, the system may fail to start due to disk missing.
    • /etc/shadow: it has appropriate permissions and can be read and written.

    Unmounting software programs

    Unmount the drivers and software that incur conflicts (including VMWare tools, Xen tools, Virtualbox GuestAdditions, and other software that comes with underlying drivers).

    Checking the virtio driver

    For more information, see Checking the Virtio Driver in the Linux System.

    Installing cloud-init

    For more information, see Installing cloud-init in the Linux System.

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

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

    Querying partitions and their sizes

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


    The returned result is similar to the following:

    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)

    From the result, you can see 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.

    The exported image should contain at least the root partition and mbr. If the exported image lacks mbr, it cannot be lauched.
    In the current operating system, if /boot and /home are independent partitions, the exported image also needs to include both independent partitions.

    Exporting an image

    You can select different methods to export an image based on your actual requirements.

    Exporting an image by using a platform tool

    For more information on how to use image export tools of VMWare vCenter Convert, Citrix XenConvert, and other virtualization platforms, see the document for the respective platform.
    Image formats supported by Tencent Cloud Service Migration are qcow2, vhd, raw, and vmdk.

    Exporting an image by running commands

    Manual export with commands poses a high risk. (For example, the file system's metadata may be corrupted when I/O is busy.) We recommend that you check the image to make sure that the image is intact and correct after it is exported.

    You can use the following commands to export an image:

    • Use the qemu-img command
      For example, 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 it to other formats, change the value of -O. Supported values are described as follows:

      Parameter ValueMeaning
      qcow2qcow2 format
      vpcvhd format
      vmdkvmdk format
      rawNo format
    • Use the dd command
      For example, run the following command to export the 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 the count parameter.
      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

      From the returned result of the fdisk command, you can see that sda1 ends at 41945087 * 512 bytes, so set count to 20481M.

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

    Converting the image format

    At present, Tencent Cloud service migration supports image formats of qcow2, vhd, vmdk, and raw. We recommend that you use a compressed image format to accelerate the transmission and migration.

    Convert the image format by using the ‘qemu-img’ command:
    For example, run the following command to convert the image from the raw format to the qcow2 format.

    sudo qemu-img convert -f raw -O qcow2 test.img test.qcow2
    • -f indicates the source image format.
    • -O indicates the target image format. For the supported formats, see -O parameter values.

    Checking the image

    The image file system that you create may be corrupted if you create the image without stopping the service or due to other reasons. Therefore, we recommend that you check the image after creating it.

    If the image format is supported by the current platform, you can directly open the image to check the 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.
    Take the Linux platform as an example:

    modprobe nbd
    qemu-nbd -c /dev/nbd0 xxxx.qcow2
    mount /dev/nbd0p1 /mnt

    If the file system is corrupted when the first partition of the qcow2 image is exported, an error occurs when you use the mount command.
    In addition, you can launch the CVM to check whether the image file works before uploading the image.

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