Logging to persistent tmpfs on Raspbian “jessie”

At the end of Using a Raspberry Pi 2 Model B as a router/firewall for the home LAN I wrote that I decided not to put /var/log into tmpfs, because:

  1. I wanted the logs to be persistent
  2. I thought that the wear would result in less and less of the sd card to become available (and 16GB for logs should last a loong time)

As it turned out the sd card died after one month.

I don’t know if the cause was excessive logging, the use of ntopng (which did write quite a lot, both in the number of files, the number of files, and in the total storage used, which was approximately 0,5GB after 30 days of uptime) or simply a bad sd card.

However, going forward with a new sd card, I’ve done the following:

  1. Removed ntopng
  2. Put /var/log on tmpfs (limited to 100MB in size), synced to a backing store on the sd card using rsync

For setting up the logging I found some existing web pages that took me part of the way, but not all the way:

Here is what I did:

  1. Logged in as root and did everything below as root
  2. Edited /etc/fstab and added the following line:
    tmpfs    /var/log    tmpfs    defaults,noatime,nosuid,mode=0755,size=100m    0 0
  3. Created an /etc/init.d/ramdiskvarlog file with the following contents
    #!/bin/sh
    ### BEGIN INIT INFO
    # Provides:          ramdiskvarlog
    # Required-Start:    $local_fs $time
    # X-Stop-After:      $time
    # Required-Start:    $local_fs $time
    # Required-Stop:     $local_fs
    # Default-Start:     S
    # Default-Stop:      0 1 6
    # Short-Description: Restore to and save logs from tmpfs filesystem
    # Description:       Restore to and save logs from tmpfs filesystem
    ### END INIT INFO
    
    # /etc/init.d/ramdiskvarlog
    #
    
    case "$1" in
      start)
        echo "Copying files to ramdisk"
        rsync -av /var/backup/log/ /var/log/
        echo [`date +"%Y-%m-%d %H:%M"`] Ramdisk Synched from HD >> /var/log/ramdisk_sync.log
        ;;
      sync)
        echo "Synching files from ramdisk to Harddisk"
        echo [`date +"%Y-%m-%d %H:%M"`] Ramdisk Synched to HD >> /var/log/ramdisk_sync.log
        rsync -avy --delete --recursive --force /var/log/ /var/backup/log/
        ;;
      stop)
        echo "Synching logfiles from ramdisk to Harddisk"
        echo [`date +"%Y-%m-%d %H:%M"`] Ramdisk Synched to HD >> /var/log/ramdisk_sync.log
        rsync -av --delete --recursive --force /var/log/ /var/backup/log/
        ;;
      *)
        echo "Usage: /etc/init.d/ramdisk {start|stop|sync}"
        exit 1
        ;;
    esac
    
    exit 0
    
  4. Made /etc/init.d/ramdiskvarlog executable:
    chmod +x /etc/init.d/ramdiskvarlog
  5. Created a directory to store the logs persistently, and populated it initially with the contents of the existing /var/log with the following command line commands :
    mkdir -p /var/backup/log
    /etc/init.d/ramdiskvarlog sync
  6. Made the /etc/init.d/ramdiskvarlog script be run at boot time and during orderly shutdown with the following command line command
    systemctl enable ramdiskvarlog
  7. Made the /etc/init.d/ramdiskvarlog script copy the contents of /var/log to the sd card once every 24 hours
    1. At the command line gave the command
      crontab -e
    2. In the editor that opened on the crontab, added a line with the following contents
      2 7 * * * /etc/init.d/ramdiskvarlog sync >> /dev/null 2>&1
  8. Created a test file with “touch /var/log/test.log”, rebooted the raspberry pi with “sync; reboot”, and then:
    1. Checked with the mount command that /var/log was on tmpfs, found the following line in the output, which meant that /var/log was on tmpfs
      tmpfs on /var/log type tmpfs (rw,nosuid,noatime,size=102400k,mode=755)
    2. Checked that the /var/log/test.log file was present (and the file was present, which meant that it had been synced to persistent storage on shutdown and restored on boot)

After completing the setup, I popped the sd card out and put it into a card reader on a debian desktop computer. Then I made an image of the working sd card, so that if/when the sd card dies, getting a working router again should be as quick as just dd’ing the image to a new sd card and then switching sd card on the raspberry Pi.

Lesson learned!

2 thoughts on “Logging to persistent tmpfs on Raspbian “jessie”

  1. Very nice script and method. Works good and I have adopted it. Before I have only created a tmpfs and recreated folders by extended scripts for logrotation, but this solves the problem that all program don’t like it when their logfile is blank, gone or missing.

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