Re: Single root filesystem evilness decreasing in 2010? (on workstations)
Clive McBarton wrote:
google "ext4 kde4" and the first hit is "Data loss may occurr when using
ext4 and KDE 4". I think Ubuntu offered ext4 as optional then and many
people ran into problems, supposedly massive data loss. XFS would be the
same. Application programmers don't cope with delayed allocation, and
since you cannot fix all the apps, you'd be stuck with the problem.
Apart from specific technical issues, there's general conservatism, most
of all in Debian.
Yep, I get that, and I know we can't possibly fix everything, but do we
really need to? In a way, all these apps are not even POSIX compliant for
the operations they intend to do, they're crappy-ext3-lazy-standards
compliant. Nobody should support that. Linux supports many filesystems,
and we shouldn't be stuck with only one for pseudo-reliable usage because a
single one of its generations had an odd behavior *some* people (not the
entire world) based their software upon. I understand people have lived
happily with XFS for a long time, which "suffers" from the same "problems".
What's worse, in my opinion, is that people feel safer with ext3 than with
ext4. What's the difference *for these apps*? Their data gets
automatically flushed every 5 seconds instead of every 30-35 seconds
(because of data=ordered). Great. If that's all that matters, let's just
change some arbitrary numbers in ext4..
I think the real buzz around this issue comes from this user who got his DE
personal conf and "some personal data" files nuked because he crashed before
they could reach the disk - which spread fear and panic in exaggerated ways
among bloggers (I read the story a hundred times). This could have happened
in ext3 with another bad timing.
Applications should tell when to sync if they need to, not rely on a
particular filesystem to do it "frequently enough" (whatever that means).
That allows software that do *not* need to (or need it less often) to be
more efficient. In some cases, a temporary file might never need to reach
the platters. Let it happen more often.
That beeing said, I tend to think that there may be fewer software which
would really need some love than we think. Note that in the meantime, there
are hacks to force flush truncated files and such that are available, which
should help apps that don't receive that love but still need it. Again, few
software probably needs some.
Paranoids about delayed allocation can also disable it. If Debian and some
other distros are and decide to do it (and/or use hacks), I certainly
wouldn't mind personally (it's not like we've ever been forced to accept
anything) but I would [mind] if we decide to hold back on ext3 - just
because it seems stupid to ignore every other features ext4 comes with,
based on a funny story on a random bug tracker.
Anyway, that topic is out of my league, and people talked about it in way
deeper depth than that at various places - I believe I understand enough of
it to make my mind. Anybody feel free to correct me if I sound misguided.
That's a very interesting point. Filesystems *not* responsible for data
integrity? Whow. While I do get the idea (move integrity checking up to
higher-level structures to improve thruput), and I am sure it will speed
things up greatly when it works, doesn't this require all your software
to first be rewritten to take care of it?
AFAIK, there has never been such a contract that filesystems should
guarantee it. In fact, they can't possibly do. If some data in file A only
makes sense if there's some specific data in file B, for example, only the
application that writes them knows - the filesystem can't detect the
corruption of the data if one file has been written but not the other.
If data integrity is important for an application, its writer should always
have the question "OK, what if it crashes *there*?" in mind and think about
an ad-hoc mechanism to make the operation atomic, in the sense that fits the
data structure of the application. If one wants some generic integrity
features, there's plenty of database software around - and by that I also
mean simple embedded/nosql stuff which could even write plain text.
Filesystem journals only guarantee that you won't get backstabbed by the
system losing or overwriting a block part of a previously coherent data
[and back on, sorry for that]
Your request is perfectly reasonable. It is clearly possible in theory,
and I believe some Unix OS actually have it (don't know which though).
It is actually required for some backup schemes (which hence don't work
Good to know.
Quick googling gave me http://lwn.net/Articles/281157/ where they say
the limitation exists up to 2.6.25 kernels (the article is from 2008
Whoa, thanks, I couldn't dig that one up. I'll go ask.
Apparently, it wasn't included in 2.6.26 (silently fails on a Lenny
machine), and I don't have a 2.6.32 kernel to play with right now; I'll try
when I can. I've now seen a few blog posts of people using it randomly
dating back from last year, even with other options - not sure if they
actually tested it, but it's a good sign. Can't find anything conclusive.
Sure. That's not fiddling with individual sectors and 3D coordinates on
the HDD, but simply using partitions at the beginning of the disk. If
you care about a factor 2, then do partition it.
Yep, sorry, what I wrote in the original post was misleading. I hadn't
anything much in mind other than that.