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Automatic device failover detection with mdadm and md_monitor

Currently, mdadm detects any I/O failure on a device and will be setting the affected device(s) to faulty. The MD array is then set to degraded, but continues to work, provided that enough disks for the given RAID scenarios are present.

The MD array then requires manual interaction to resolve this situation.

  1. If the device had a temporary failure (eg connection loss with the storage array) it can be re-integrated with the degraded MD array.
  2. If the device had a permanent failure it would need to be replaced with a spare device.

1) Automatic device integration after temporary failure

The md_monitor program has been developed to handle case 1), ie automatic reintegration of a device after a temporary failure.

To facilitate this the md_monitor program monitors each MD array of the system. For each device which is part of an MD array a checker thread is started. This checker thread issues a READ request to sector 0 of the device in regular intervals. Depending on the current status of the device and the result of the I/O it will then take corrective action:

a) I/O returned ok:

  • if the MD array has registered the device as ready, no action is triggered.

  • if the MD array has registered the device as faulty, a manual array recovery is triggered by executing the command:

    mdadm --manage /dev/mdX --remove /dev/dasdY --re-add /dev/dasdY

b) I/O returned with error:

  • if the MD array has registered the device as faulty, no action is triggered.

  • if the MD array has registered the device as ready, the MD array is instructed to fail the device by executing the command:

    mdadm --manage /dev/mdX --fail /dev/dasdY

c) I/O does not return after a given timeout:

  • if the number of retries has not been exceeded, re-check after waiting one second
  • if the number of retries has been exceeded, treat the device as if an I/O error had happened and continue as in 1.b)

2) Device replacement after a permanent failure

The md_monitor program continues to issue I/O to a device even if the device is failed. On S/390, the DASD subsystem does not allow for a device to be deactivated if there is any I/O pending. So to replace a device after a permanent failure the md_monitor program has first to be instructed to stop monitoring the device before it can be replaced.

To do this, the administrator has to issue the command:

    /sbin/md_monitor -c "Remove:/dev/mdX@/dev/dasdY"

to instruct md_monitor to stop monitoring device /dev/dasdY on MD array /dev/mdX. Then the device can be set to faulty with

    mdadm --manage /dev/mdX --fail /dev/dasdY

and removed from the MD array with:

    mdadm --manage /dev/mdX --remove /dev/dasdY

The new disk can be added with

    mdadm --zero-superblock /dev/dasdZ
    mdadm --manage /dev/mdX --add /dev/dasdZ

md_monitor will pick up the changes automatically and start monitoring the new device.

3) Set-up md_monitor: simple setup with systemd

  1. Make sure the number of system asynchronous IO slots is high enough for md_monitor (only necessary on SLE12, with kernel below 4.4.155-94.50.1):

    echo "fs.aio-max-nr=$((1<<20))" >/etc/sysctl.d/99-aio.conf

  2. Set MDADM_PROGRAM in /etc/sysconfig/mdadm:

    MDADM_PROGRAM="/usr/share/misc/md_notify_device.sh"

  3. Customize the command line options for md_monitor in /etc/sysconfig/md_monitor to suit your system's needs.

  4. Enable the md_monitor service:

    systemctl enable md_monitor

  5. Reboot to make sure all settings take effect.

4) Set-up md_monitor: detailed instructions

Make sure the number of system aio slots is high enough for md_monitor (see above).

md_monitor is informed about state changes from MD array either from uevents or from mdadm in monitor operation. mdadm needs to be started with

mdadm --monitor --scan --program <MONITOR_SCRIPT>

where <MONITOR_SCRIPT> is a bash script containing the following:

#!/bin/bash
# MD monitor script
#

EVENT=$1
MD=$2
DEV=$3

/sbin/md_monitor -c "${EVENT}:${MD}@${DEV}"

Assuming the md_monitor program has been installed under /sbin. The default monitor script is installed under

/usr/share/misc/md_notify_device.sh

5) md_monitor Documentation

md_monitor has the following command-line options:

-d, --daemonize Start md_monitor in background

-f <file>, --logfile=<file> write logging information into <file> instead of stdout

-s, --syslog Write logging information to syslog.

-e <num>, --expires=<num> Set failfast_expires to <num>

-r <num>, --retries=<num> Set failfast_retries to <num>

-p <prio>, --log-priority=<prio> Set logging priority to <num>

-v, --verbose Increase logging priority

-c <cmd>, --command=<cmd> Send command <cmd> to daemon

-h, --help Display usage information

The --command option instructs the program to connect to an already running md_monitor program and send a pre-defined command. The command has the following syntax:

<cmd>:<md>(@<dev>)

The following values for <cmd> are recognised. If not specified otherwise, <md> needs to be the device node of an existing MD array.

Shutdown Shutdown md_monitor; <md> argument should be /dev/console.

RebuildStarted Rebuild has started on array <md>.

RebuildFinished Rebuild has finished on array <md>.

DeviceDisappeared MD array has been stopped; md_monitor will stop monitoring of the component devices on array <md>.

Fail MD detected a failure on the component device <dev>. md_monitor will re-check the device every failfast_timeout seconds.

Remove The component device <dev> has been removed from the MD array <md>. md_monitor will stop monitoring this device.

SpareActive MD has integrated the device <dev> into array <md>. md_monitor will re-start monitoring this device every failfast_timeout seconds. The check interval will be increased for each successful check up to a maximum of failfast_timeout * failfast_retries seconds.