System Clock on the PandaBoard

A quick way to find the physical device backing a filesystem is to use the 'df' command.  (LVM, software-RAID, LUKS, and other layers can make this more complicated.)  I did this on my PandaBoard and was a little surprised at the answer:

[root@fedora-arm ~]# df -Th /
Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/root      nfs    7.4G  1.8G  5.3G  25% /

Why is it NFS-mounting a USB flash drive?  And /dev/root doesn't tell me what the device name is.  I was expecting to see something like sda1 or mmcblk0p1.

I thought that was weird, so I edited /etc/fstab and changed the device and filesystem type:

[root@fedora-arm ~]# diff -u /etc/fstab.ORIG /etc/fstab
--- /etc/fstab.ORIG        2011-11-14 10:09:40.000000000 -0600
+++ /etc/fstab  2011-11-14 12:48:58.000000000 -0600
@@ -1,4 +1,4 @@
-/dev/root               /                       nfs     defaults        1 1
+/dev/sda1               /                       ext3    defaults        1 1

And then I rebooted my PandaBoard and quickly learned why it's set to NFS:

                Welcome to Fedora
                Press 'I' to enter interactive startup.
Starting udev: [  OK  ]
Setting hostname fedora-arm:  [  OK  ]
Setting up Logical Volume Management:   No volume groups found
[  OK  ]
Checking filesystems
Checking all file systems.
[/sbin/fsck.ext3 (1) -- /] fsck.ext3 -a /dev/sda1
fedoraroot: Superblock last mount time (Mon Nov 14 11:19:35 2011,
        now = Sat Jan  1 00:00:01 2000) is in the future.


fedoraroot: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
        (i.e., without -a or -p options)
[FAILED]

There's no battery-backed clock on the PandaBoard so the clock gets reset to 2000-01-01 00:00:00 every time it's power-cycled, and this causes the Fedora boot scripts to get confused and force a filesystem check.

Apparently mounting the root filesystem as NFS is one way to workaround  this problem.

I mentioned this in #fedora-arm on freenode and Chris Tyler suggested modifying the boot scripts to set the clock to 1 minute past the previous write time of the root filesystem.

Here's what I came up with:

--- /etc/rc.d/rc.sysinit.ORIG    2011-03-08 15:51:46.000000000 -0600
+++ /etc/rc.d/rc.sysinit    2011-11-14 12:44:28.441405990 -0600
@@ -577,6 +577,15 @@
 fi
 if [ -z "$fastboot" -a "$READONLY" != "yes" ]; then

+        # set the clock on the PandaBoard to 1 minute after last write
+        # time of root filesystem to avoid "last mount is in the future"
+        echo "Current system clock is $(date)"
+        rootdev=$(egrep '\s/\s' /etc/fstab | awk '{print $1}')
+        lastwrite=$(dumpe2fs -h $rootdev 2>/dev/null | \
+                    grep "Last write time" | sed -e 's/^[^:]*:\s*//')
+        echo "Setting system clock to 1 minute after $lastwrite"
+        date -s "$lastwrite + 1 minute"
+
         STRING=$"Checking filesystems"
         echo $STRING
         fsck -T -t noopts=_netdev -A $fsckoptions

Notice the new messages (red text) during boot:

                Welcome to Fedora
                Press 'I' to enter interactive startup.
Starting udev: [  OK  ]
Setting hostname fedora-arm:  [  OK  ]
Setting up Logical Volume Management:   No volume groups found
[  OK  ]
Current system clock is Sat Jan  1 00:00:01 CST 2000
Setting system clock to 1 minute after Mon Nov 14 12:45:36 2011
Mon Nov 14 12:46:36 CST 2011
Checking filesystems
Checking all file systems.
[/sbin/fsck.ext3 (1) -- /] fsck.ext3 -a /dev/sda1
fedoraroot: clean, 69076/489600 files, 487736/1957605 blocks
[  OK  ]
...

And the root filesystem and device look correct now!

[root@fedora-arm ~]# df -Th /
Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/sda1     ext3    7.4G  1.8G  5.3G  25% /

Note that I do have ntpd start later in the boot process which sets the actual correct time.