Udev rules are getting reloaded frequently and causing Oracle ASM outage

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Environment

  • Red Hat Enterprise Linux 6
  • Red Hat Enterprise Linux 7
  • Red Hat Enterprise Linux 8
  • Oracle DB using ASM disks managed by udev

Issue

  • Udev rules are getting reloaded frequently and causing Oracle ASM outage
  • After installing tuned on system, we are getting high SYS CPU at specific times when the tuned-mpath-iosched rule is triggered from /lib/udev.

Resolution

  • There was a known issue with inotify watch events configured in the udev rules. Due to this, when a process opens a device for write operation, and then close it, then it could synthesise a change event. And this change event reloads the udev rules configured with ACTION=="add|change".

    The udev man page describes the watch and nowatch options for 'inotify':

    $ man udev
    [...]
       watch
           Watch the device node with inotify, when closed after being opened for writing, a change uevent will be synthesised.
    
       nowatch
           Disable the watching of a device node with inotify.
    [...]
    

    The 'inotify' watch and change event is turned on for most of the devices:

    $ fgrep watch -r /lib/udev/rules.d/* | fgrep OPTIONS
    lib/udev/rules.d/10-dm.rules:OPTIONS:="nowatch"
    lib/udev/rules.d/11-dm-lvm.rules:OPTIONS:="nowatch"
    lib/udev/rules.d/13-dm-disk.rules:OPTIONS+="watch"
    lib/udev/rules.d/60-persistent-storage.rules:KERNEL!="xvd*|sr*", OPTIONS+="watch"
    lib/udev/rules.d/60-persistent-storage.rules:KERNEL=="xvd*", ENV{DEVTYPE}!="partition", ATTR{removable}!="1", OPTIONS+="watch"
    lib/udev/rules.d/60-persistent-storage.rules:KERNEL=="xvd*", ENV{DEVTYPE}=="partition", OPTIONS+="watch"
    lib/udev/rules.d/64-md-raid.rules:OPTIONS+="watch"
    lib/udev/rules.d/80-udisks.rules:# KERNEL=="dm-*", OPTIONS+="watch"
    

    To suppress the false positive 'change' events, disable the 'inotify' watch for the devices used by Oracle ASM:

    [1] Create a udev rule file /etc/udev/rules.d/96-asm-device.rules and set nowatch option:

    ACTION=="add|change", KERNEL=="sd*", OPTIONS:="nowatch"
    

    NOTE: If you're using ASM on multipath devices, you'll need to change the "sd" to "dm" as shown in the following example:

    ACTION=="add|change", KERNEL=="dm-*", OPTIONS:="nowatch"
    

    [2] Then use below commands to reload the udev rules configuration:

    $ /sbin/udevadm control --reload-rules
    $ /sbin/udevadm trigger --type=devices --action=change
    
    Note:

    o Above commands will reload the complete udev configuration and will trigger all the udev rules. On a busy production system this could disrupt the ongoing IO operations. So it would be recommended to please use the above commands during a scheduled maintenance window only.

    o The udev rule shown in above snip is provided as an example, there may be further change required to accommodate the devices used by Oracle ASM. Please get in touch with Red Hat support representative for assistance in setting up the udev rules to selectively disable the 'inotify' watch for the devices used by Oracle ASM.

    o The 'nowatch' option would only disable the 'inotify' watch events. The inotify API provides a mechanism for monitoring the filesystem events. It is used to monitor the individual files or to monitor directories. So, setting the 'nowatch' option for devices does not disable any existing device specific udev rules and any changes to underlying disk (e.g. add/remove/change) would still trigger the udev rules as expected.

Root Cause

  • Monitoring the udev events using below commands showed that there were frequent 'change' events triggered for the devices and the rules were getting reloaded.

    $ udevadm monitor --property  &>>  /tmp/udevadm_monitor_property
    $ udevadm monitor --kernel  &>>  /tmp/udevadm_monitor_kernel
    $ udevadm monitor --udev  &>> /tmp/udevadm_monitor_udev
    
  • Here is an example of an open test case with no write triggering the rule

    ./check_udev /dev/mapper/mpathmn
     Opened file /dev/mapper/mpathmn with descriptor 3
    
    KERNEL[1433182713.102117] change   /devices/virtual/block/dm-1 (block)
    UDEV  [1433182713.823458] change   /devices/virtual/block/dm-1 (block)
    
    'check_udev.c' program used for binary 'check_udev':
     #include <stdio.h>
     #include <stdlib.h>
     #include <sys/types.h>
     #include <sys/stat.h>
     #include <fcntl.h>
    
     int main(int argc, char*argv[])
     {
     int fd;
     fd=open(argv[1],O_RDWR,0660);
     printf("Opened file %s with descriptor %d\n",argv[1],fd);
     close(fd);
     }
    
  • While investigating the reason for 'change' events generated during device open/close operations, it was found that there is a 'inotify' watch enabled through couple of rules:

    $ fgrep watch -r /lib/udev/rules.d/* | fgrep OPTIONS
    lib/udev/rules.d/10-dm.rules:OPTIONS:="nowatch"
    lib/udev/rules.d/11-dm-lvm.rules:OPTIONS:="nowatch"
    lib/udev/rules.d/13-dm-disk.rules:OPTIONS+="watch"
    lib/udev/rules.d/60-persistent-storage.rules:KERNEL!="xvd*|sr*", OPTIONS+="watch"
    lib/udev/rules.d/60-persistent-storage.rules:KERNEL=="xvd*", ENV{DEVTYPE}!="partition", ATTR{removable}!="1", OPTIONS+="watch"
    lib/udev/rules.d/60-persistent-storage.rules:KERNEL=="xvd*", ENV{DEVTYPE}=="partition", OPTIONS+="watch"
    lib/udev/rules.d/64-md-raid.rules:OPTIONS+="watch"
    lib/udev/rules.d/80-udisks.rules:# KERNEL=="dm-*", OPTIONS+="watch"
    

    The OPTIONS+="watch" causes udev to watch the device with 'inotify'.

    o So, if any program opens a disk device in writable mode with open() and close() it, then udev synthesises a 'change' uevent. This is because, if the disk is modified, the disk label, UUID, or partitions or anything else for the disk could have changed. The udev daemon has to rescan the disk and update the udev database accordingly.

    o Due to this change event on disk devices, any udev rules having ACTION=="add|change" will get reloaded.

Diagnostic Steps

  • Run following commands from separate terminals and monitor the output to verify if there are any frequent change events triggered for the disks:

    $ udevadm monitor --property
    $ udevadm monitor --kernel
    $ udevadm monitor --udev
    

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