LVM negates benefits of jounaling filesystems? [was RFE: autofsck]

[Eric Sandeen]

Yaakov Nemoy wrote:

> This means, your system crashes while doing a banking transaction, you
> don't lose money without a record being made to whom you paid it to.
> It means that while rotating a photo, your power goes out, you don't
> have half a rotate photo saved on your hard drive.

Well, not in general, at least not from normal filesystem journaling.
The stuff you're talking about here is by and large the responsibility
of the app doing the writing.  Filesystem journaling is often only for
metadata, to ensure metadata consistency, or something like
data=journaled... but if your photo app writes out half of the edited
jpeg over the top of the existing file in one syscall and then you crash
before it makes a 2nd intended write, in general journaling is not going
to help you there, either.

see for example this talk:

eat my data: how everybody gets file IO wrong
http://lca2007.linux.org.au/talk/278.html

We'll discuss:
- when data hits disk
- what causes data to hit disk
- how you can ensure that data is on disk
- how applications get this wrong
- how a file system is not a database (and vice versa)

-Eric