My home machine has a sufficiently old set of filesystems that many of my actively used filesystems are still ext3, not ext4, including both my home directory and where I keep code. Normally this isn't something that I particularly think or worry about; it's not like ext4 is a particularly radical advance from ext3 (certainly not the same sort of jump that was ext2 to ext3, where you got fast crash recovery). As a sysadmin I'm generally cautious with filesystem choices anyways (at least when I'm not being radical); I used ext2 over ext3 for years after the latter came out, for example, on the principle that I'd let other people find the problems.

It turns out that there is one important thing that ext4 has and ext3 does not: ext4 has sub-second file timestamps, while ext3 only does timestamps to the nearest second. Modern machines are fast enough that nearest second timestamps are increasingly not really good enough when building software or otherwise doing things that care about relative file timestamps and 'is X more recent than Y'. Oh, sure, it works most of the time, but every so often things go wrong or you find assumptions buried in other people's software.

Most people don't notice these things because most people are now using filesystems that support sub-second file timestamps (which is almost all modern Linux filesystems). What this tells me is that I'm increasingly operating in an unusual and effectively unsupported environment by continuing to use ext3. As time goes by, more and more software is likely to assume sub-second file timestamps basically by default (because the authors have never run it on a system without them) and not work quite right in various ways. I can fight a slow battle against what is effectively a new standard of sub-second file timestamps, or I can give in and convert my ext3 filesystems to ext4. It's not like ext4 is exactly a new filesystem these days, after all (Wikipedia dates it to 2008).

The mechanics of this conversion raise a few issues, but that's something for another entry.