Ubuntu – No entry in fstab but partition is mounted at bootup

Something is very fishy about that fstab-line:

/dev/xvdf /dev/xvdb1 ext3 defaults 0 0

Normally they have the format:

/dev/device /mnt/mountpoint fs flags stuffINeverRemember

Basically, what this line does or tries is mounting /dev/xvdf onto the directory /dev/xvdb1. I think you misunderstood how /etc/fstab works. You most likely want something like this:

/dev/xvdb1 /mnt/ ext3 defaults 0 0

In particular, note that the mount point (/mnt in this example) must be an existing directory, it will not be created for you.

All three data journaling modes should leave the filesystem itself fully intact after a power failure. So it should always mount without errors. The difference is only in the data in your files; data=writeback mode may leave stale data (i.e., what was stored in the disk sectors before the writes your app did). data=ordered and data=journaled should not do this.

Most likely what you're seeing is that I/O barriers aren't working on your setup. First, make sure you're not mounting with barrier=0/nobarrier. That boosts performance, but will cause corruption on power failure.

If I/O barriers are on, it's also possible you're passing through a storage layer that doesn't support them. On older releases, LVM didn't and various mdraid levels didn't. (This was fixed in Linux 2.6.33; so only if you're running Lucid still.)

Finally, it's possible your disks are telling lies. Disks have write caches. Especially with NCQ, they're supposed to only tell the OS they've written data when they've actually done so, but they've been known to tell the OS its written when its only in the disk's write cache. Increases performance. At least as long as the power stays on. You can try disabling the write cache on the disks, though you'll take a performance hit for this.

Note also that flash-memory disks have a lot of work to do under the hood, and many of them don't handle power failure well. (For example, wear leveling sometimes requires that a full flash block of data be moved. If the power fails in the middle, bad things happen on some flash disks.)

Finally... have you considered an UPS?