Linux – Block device naming, how to enable by-name and by-uuid

As with all things pertaining to security, there aren't any guarantees, but you also need to balance risk (and cost) against probability. From experience (and I've been running dozens of *nix boxen since the dark ages), I've never really had significant power-caused filesystem corruption.

Some of these machines were even running on non-journalled filesystems (ufs and ext2 usually). Some of them were embedded, and a few were mobile phones like the Nokia N900 — so a good power supply wasn't at all guaranteed.

It's not that filesystem corruption can't happen, it's just that the probability of it happening is low enough that it shouldn't worry you. Still, no reason not to hedge your bets.

In answer to your literal questions:

1. At least the first book you referenced was written before ext4 — when the author suggests using ext3, they're really saying ‘don't use unstable or non-journalled filesystems like ext2’). Try ext4, it's quite mature, and has some decent options for non-spinning disks which may extend the life expectancy of your flash device.
2. Chances are it would lose you the last block or two, not the entire file. With a journalled filesystem, this will be about the only loss. There are failure scenarios where I could see random data sprayed across the file, but they seem about as likely as a micrometeorite smashing right through your embedded device.
3. See 2. Nothing is 100.00% safe.

4. If you have a second IDE channel, stick a second CF card in there and grab a backup of the filesystem periodically. There are a few ways to do this: rsync, cp dump, dd, even using the md(4) (software RAID) device (you add the second drive occasionally, let it sync, then remove it — if both devices are live all the time, they run the same risk of filesystem corruption). If you use LVM, you can even grab snapshots. For a data collection embedded device, I'd just use am ad hoc solution which mounts the second filesystem, copies over the data log, the immediately unmounts it. If you're worried about the device having a good boot image, stick a second copy of the boot manager and all necessary boot images on the second device and configure the computer to boot from either CF card.

   I wouldn't trust a second copy on the same device because storage devices fail more often than stable filesystems. Much more often, in my experience so far (at work, there was a bitter half-joke about the uncannily high chances of Friday afternoon disk failures. It was almost a weekly event for a while). Whether the disk is spinning or not, it can fail. So keep your eggs in two baskets if you can, and you'll protect your data better.

   If the data is particularly sensitive, I'd pay regular visits to the device, swap the backup CF for a fresh one and reboot, letting it fsck all its filesystems for good measure.

If you specify a block size (512 bytes) of less than the block size of the disk (often 4096 bytes, but nowadays maybe more), the block will be partially written, so that the contents of the rest of the block must be preserved before writing.

This is because disk blocks cannot be written to with only 512 bytes, but you have to write a full block at once (4096 or larger). When you write this (4096) amount or more, there is no partial write, so it does not have to read.