Is ext4 Stable For Production Systems?

dr_dracula writes "Earlier this year, the ext4 filesystem was accepted into the Linux kernel. Shortly thereafter, it was discovered that some applications, such as KDE, were at risk of losing files when used on top of ext4. This was diagnosed as a rift between the design of the ext4 filesystem and the design of applications running on top of ext4. The crux of the problem was that applications were relying on ext3-specific behavior for flushing data to disk, which ext4 was not following. Recent kernel releases include patches to address these issues. My questions to the early adopters of ext4 are about whether the patches have performed as expected. What is your overall feeling about ext4? Do you think is solid enough for most users to trust it with their data? Did you find any significant performance improvements compared to ext3? Is there any incentive to move to ext4, other than sheer curiosity?"

Comments

Risk Vs Benefits Analysis

[eldavojohn]

Is ext4 Stable For Production Systems?

Probably.

Is there any incentive to move to ext4, other than sheer curiosity?

Ok so I'm gussing production = income = your ass? Let me turn your question back to you by asking, "What is driving this need to move to ext4?" Because so far, all you've told me is that you are considering risking your ass for sheer curiosity.

I may be grossly misinformed but that is how the question sounds to me. And by "your ass" I don't mean oh-no-we-had-a-service-outage-for-five-minutes ... no, we could have a customer on the phone saying, "You mean to tell me that the modifications being made to my site for the past 24 hours are gone?!"

If it ain't broke, don't fix it!

I don't know about you but I'm too busy dealing with shit like this than to ponder new potential problems I can put into play.

Look through this page for a rough comparison of ext4 with other file systems. There's a better list of features for ext4 here that will tell you why you might need to switch to it. It is backward compatible with ext3 and ext2 so moving to it may be trivial. If you're dealing with more than 32000 subdirectories or need to partition some major petabytes/exobytes then you might not have a choice. Some of these benefits are probably not risking your ass for but if there's a business need that cannot be overcome any easier way then back your shit up and do rigorous testing before you go live with it. If you're using Slashdot to feel out if the majority of users scream OMGNOES so you don't waste your time doing that, then that's fine. Just don't do this if you don't have to.

I tell you what, there's a $288 desktop computer at Dell today that you can buy, put ext4 on and your OS of choice and your application(s) and whipping boy it into next century without risking anything. Where I work we have two servers in addition to our production servers. I don't think this is an uncommon scheme so if you have a development server, throw it on there and poke it with a stick. Then move it to the testing server and let your testers grape it for two weeks. Then you'll know.

Ye

[identity0]

I've been running ext4 on my system and everything's fi

Wrong question

[AmiMoJo]

You are asking the wrong question. Ext4 does not need fixing, the apps do.

Are your apps patched yet?

Re:Wrong question

[QuoteMstr]

Face it: your side lost. "fsync everywhere" is an infeasible, untenable, and useless position to take.

fsync-on-rename creates a much better environment for application developers and users alike. The Right Thing happens by default, and I maintain that nobody actually wants the unsafe rename behavior. Allowing an application "choice" in this respect is a red herring.

The only improvement I'd make it to flush the file involves on every rename, not just renames that happen to overwrite an existing file. Under the current scheme, an application doing the write-close-rename to replace a file will still be put in a bind if the file to write doesn't exist yet. (i.e., you can still end up with a zero-length file where no such file ever existed on a running system)

Re:Wrong question

[k8to]

There was no single loser here.

Ext4 should handle the case gracefully, but the apps will fail on other filesystems, and they *will* be run on those filesystems, so they should fix the bugs.

Re:Wrong question

[blueg3]

The problem is that some applications assume a behavior that is not supported by the POSIX definitions (the guarantees provided by the OS functions they're calling). However, it happens to be the behavior on existing filesystems and happens to be convenient. Now a new filesystem comes along and sticks to the POSIX definitions but does not follow this behavior. Application breaks, people complain.

As a simplified example, imagine you create file B, then delete file A. Existing filesystems happen to do this in order, so you always have at least one of A or B. (If the system crashed partway through, you might have both A and B.) Your application fails if neither A nor B is present. POSIX doesn't require that the operations be performed in order. New filesystem comes along and sometimes does them in the reverse order, so if the system crashes at the wrong time, neither A nor B is left on the filesystem.

Re:Wrong question

[Anonymous Coward]

Only on Linux is it the user's fault that apps have data loss because the Linux kernel people changed filesystem semantics. At least Microsoft takes some responsibility for their mistakes :-/

I did follow the ext4 debate. Here's my quick synopsis.

• Linux kernel hacker discovers he can make a certain microbenchmark run 50% faster if he allows reordering of filesystem metadata writes ahead of filesystem data writes. Said hacker checks in code with a "now 50% faster!!!" message.
• A few months later, users start discovering data corruption of KDE files. Specifically, a copy of A to A', ftruncate(A'), write(A'), rename(A' to A), host crash, causes the resulting file to contain A data and not A' data despite the well-known atomic "rename" that serves as a barrier.
• Linux kernel hacker ignored problem as not-a-bug, since the apps didn't make use of fdatasync() / fsync() correctly, which (using Posix semantics) would have prevented data corruption. The detail to note here is that Posix doesn't actually say that rename is a write barrier for data and metadata, even though everyone would assume that it is a write barrier and ALL other filesystems have treated it as a write barrier. (And in my opinion as a professional systems programmer, this is an oversight in the Posix standard and not a desired behavior). So the linux kernel hacker is technically correct but has introduced a behavior that goes against all previous implementations.
• Linux kernel hacker (and some Slashdot posters) attack KDE developers for being incompetent because they didn't read a sub-sub-sub clause of the Posix spec that (1) isn't mentioned in the man pages, (2) only gets read by kernel programmers anyway, and (3) is about two orders of magnitude more arcane than the average desktop app developer will ever read documentation.
• 90% of users and 80% of programmers wonder what the hell fdatasync() and fsync() and the difference between data and metadata write barriers are, and why the default behavior is to corrupt data.
• Linux kernel hacker promises to commit a few patches to fix the problem, so as not to break software that has worked perfectly fine for the past 10 years.
• Those of us with experience realize that since said kernel hacker didn't believe this was a problem in the first place, the patches are as likely to be half-hearted band-aids as to actually increase data integrity guarantees. Programming has a long and proud history of making a quick fix to satisfy "management" (in this case, the Linux community) that makes one symptom go away and doesn't actually fix the underlying problem.
• We get an Ask Slashdot asking if the problem actually got fixed, because 99% of us do not have the technical expertise to understand patches to the Linux filesystem to figure out if this actually got fixed.

I do have a moral to this story. Filesystems have one cardinal, inviolable rule. DO NOT CORRUPT THE USER'S DATA. The guarantee is that if a user makes a read, the user will get back either good data OR an error (or explicit indication of no data). Google likes filesystems that lose data - but they don't ever give back corrupt search results. Ext3 can reorder writes - but defaults to a safe 5-second flush rate to keep the window of unexpected corruptions small. Ext4 ignored this rule and allows silent data corruption so that this filesystem can be the best at certain microbenchmarks, and instead of accepting responsibility, the kernel hacker in question blames everybody else.

The greatest danger to Linux's success is not Microsoft. It's the hubris of many Linux developers, users, and advocates, who are too busy disavowing responsibility and blaming everybody else to fix real user's problems. (And yes, I'm a follower of the Raymond Chen philosophy)

Re:Wrong question

[Jane Q. Public]

Huh? Buddy, this is Slashdot. There are lots of single losers here.

Re:Wrong question

[QuoteMstr]

But even then you might end up with a zero byte file, if your system crashes between the close and rename call. (Or between write and close, or doing write, or well anytime after open).

This statement is incorrect. Suppose you want to atomically replace the contents of file "foo". Your application will write a file "foo.tmp", then call rename("foo.tmp", "foo"). At no time on a running system does any process observe a file called "foo" that does not have either the new or the old contents, and this invariant holds true whether or not "foo", "foo.tmp", or any other file has been flushed to the disk.

On the filesystem level, the kernel can actually write the contents of foo.tmp to disk whenever is convenient. The only constraint is that the on-disk name record for "foo" must be updated to point to the new data blocks from foo.tmp only after these data blocks have themselves been written to disk. That's the issue here: without that ordering guarantee, the kernel can write a file's name record before its data blocks. If the system crashes after the name record is written but before the data blocks are, what's observed on the recovered system is a zero-length file.

That's the problem here: the kernel is conjuring out of thin air a zero-length file that never actually existed on a running system.

Forcing applications to call fsync is not only an onerous burden on application developers, but it also reduces performance because it gives the filesystem less freedom than the much looser constraint on rename above.

Bonus points for anyone who can give a realistic use case for DO_NOT_FLUSH_ON_CLOSE

1. Application configuration files. You don't care that they hit the disk immediately, but only that when they do hit the disk, they're not corrupt
2. /etc/mtab

Flushing on close is the wrong thing: it far exceeds the minimum requirements that most applications actually need, which will substantially reduce performance.

Re:I think it's "safe enough"

[eldavojohn]

I moved to ext4 as soon as it became available. I haven't had any problems thusfar (no data loss, etc), and the increased speed is noticable. So - in the opinion of a very casual Linux user - I would say that yes, it's "okay." I'm not sure I'd trust it with anything super serious, though. I could be the only one without any problems, after all. As always, you should tip-toe around anything bleeding-edge.

Yeah, man, it's ok go ahead and flip your entire corporation's servers to ext4 over this weekend. A Slashdot user named buttfscking just said it is "safe enough."

Um, yes, it's called fsck.

[dandaman32]

I'm using ext4 on an encrypted partition on my tiny X41 tablet. The hard disk is 5400RPM IIRC, so when Ubuntu decides to run fsck due to a scheduled run or an unclean shutdown after a certain bug manifests itself, I don't have to sit there for 10 minutes or more waiting for fsck to run. That for me and many other casual users is probably the biggest advantage of ext4.

Does a laptop count as production? In the eyes of an everyday user, yes. My laptop is very much "production" IMHO, and I trust ext4 enough to not magically make all my school assignments disappear.

Digressing a bit, I haven't seen any of the data loss either, though I use GNOME and not KDE. I do think that if an application relies on specific undocumented behavior, that the application should change, not the filesystem driver. It's acceptable that the kernel developers are doing their best to get temporary workarounds into place, but the permanent solution is to fix the applications so they don't depend on undocumented behavior.

Theodore Ts'o: Donâ(TM)t fear the fsync!

[sirdude]

After reading the comments on my earlier post, Delayed allocation and the zero-length file problem as well as some of the comments on the Slashdot story as well as the Ubuntu bug, itâ(TM)s become very clear to me that there are a lot of myths and misplaced concerns about fsync() and how best to use it. I thought it would be appropriate to correct as many of these misunderstandings about fsync() in one comprehensive blog posting.

http://thunk.org/tytso/blog/2009/03/15/dont-fear-the-fsync/

FYI, Ts'o is the ext4 maintainer.

Re:ext4 is buggy

[TCM]

But he uses R-A-I-D! R-A-I-D magically makes data bulletproof and immune to disaster as we all know.

Seriously, running a 3TB RAID with a buggy fs and applauding faster fsck times instead of wondering why the fs gets fucked up constantly must be the peak of idiocy.