Article ID: 122106, created on Jun 20, 2014, last review on Jun 20, 2014

  • Applies to:
  • Virtuozzo
  • Virtuozzo hypervisor


  1. File system inside a Linux virtual machine was remounted in read-only mode.
  2. The remount happened during high load on both host and guest operating systems (e.g during the transfer of large amount of data).


The problem could have been caused by plain file system corruption.

In case the problem happened during high load on both host and the virtual machine, it could also could have been caused by double reordering of disk input/output requests: the host reorders the I/O requests to maximize the performance, keeping the data in memory if possible, while the guest system has NCQ support enabled on disks and tries to optimize the access as well. Due to reordering on the host side, some requests are delayed and the guest operating system registers it as the disk access fault. Hitting the fault in journal area leads to immediate remounting of the file-system in read-only mode.


  1. Reboot the virtual machine in single user mode
  2. Run e2fsck to check and repair the file system

Also it is recommended to disable NCQ feature for the virtual machine:

  1. Stop the virtual machine
  2. Add the devices.ahci.ncq=0 boot flag to the virtual machine's configuration.

    Boot flag can be added using either of the following ways:

    • Parallels Management Console (Select the VM -> Configure -> Hardware -> Boot Order -> Boot flags field)
    • Parallels Virtual Automation (Select the VM -> Hardware Settings -> CPU -> System flags field)
    • Manually modifying the virtual machine's configuration file (config.pvs):

      # prlctl list -i TestVM | grep Home
      Home: /var/parallels/TestVM.pvm/
      # grep SystemFlags /var/parallels/TestVM.pvm/config.pvs
  3. Start the virtual machine

Search Words

disable ncq

IO error in the VM

Buffer I/O on device

ro filesystem

aborting journal

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