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.

Re:Risk Vs Benefits Analysis

[Joce640k]

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

This.

Re:Risk Vs Benefits Analysis

[BrokenHalo]

A shorter approach to the question:

What do I gain by running with ext4?
Is that gain worth the time spent changing what I've got?

If the answer to the first question is that ext4 is cool and shiny, and the answer to the second is unknown, the OP has his answer.

Filesystems are one thing we need to be VERY conservative about. We need to be certain that it works reliably, because we do not need to find our work disappearing out the end of our backup cycle after having discovered problems too late. (Yes, I know, what is this "backup" of which I speak?)

I still have drives running ReiserFS, and I still use ext2 for boot partitions mounted readonly. I pretty much trust those systems, but even so, I still take backups and test them when I can.

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

[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

[eldavojohn]

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

Are your apps patched yet?

At the risk of revealing just how incredibly inept I am about file systems ... shouldn't your "apps" (and by apps I am guessing you mean applications) be calling the operating system to do anything to the file system? I mean, isn't the point of operating systems to create or contain APIs and the like that allow you to interface with any file system type that the OS supports?

I guess what I'm asking is just the technicality that only his operating system need be patched and tested for it?

Again, I don't really do this type of coding and in all the C programming I've done, I've never seen a need or way even to get down and dirty with the file system. I can dream up cases (like Google's bigtable) where that may be desirable with benefits if well planned but I would imagine most of the time it would be unwise and unsafe and put you dependent on a type of file system.

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

[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:Wrong question

[QuoteMstr]

The problem is that some applications assume a behavior that is not supported by the POSIX definitions

POSIX is a red herring here. It covers the behavior of a running system, and makes no guarantees about atomicity or durability following a crash. After a crash and as far as POSIX goes, it's perfectly legitimate to overwrite the entire disk with hentai. Every crash recovery technique goes beyond POSIX because POSIX says nothing about crashes.

POSIX doesn't require that the operations be performed in order

It most certainly does! On a running system, if you rename B over A, at no point does any process on the system observe a file called "A" that does not have either the contents of the old A or the contents of B. THIS ATOMICITY IS A FUNDAMENTAL POSIX GUARANTEE.

Filesystems should do their best to honor this guarantee (which always applies on a running system, remember) even when the system crashes. Filesystems don't have to do that according to POSIX. Instead, they should do it because it's a sane thing to do, and doesn't violate anything POSIX guarantees. POSIX is not the arbiter of what a good system should be. It's perfectly reasonable to make guarantees that go beyond POSIX, and every real-world operating system does precisely that. POSIX guarantees are necessary but insufficient for a reasonable system in 2009.

Re:Wrong question

[QuoteMstr]

Nor is fsync() what you want - you want an atomic file replace operation.

Yes.

Rename is atomic, and it used to work, but with delayed allocation it may happen before the file is written. So what you want is an atomic file replace operation that does not happen before the data write.

Precisely.

Rename may not be the best option for that - a special file write mode may actually be better. In any case the issue affects both sides - kernel and user space.

NO, NO, NO. write, fsync, close, rename is how you spell "atomically replace this file" in terms of system calls. It does precisely the correct thing on a running system. You yourself admit that it "used to work". It has worked for decades, in fact. (Though before journaling filesystems, all bets were off after a crash.)

That sequence of system calls is how applications tell the kernel to replace the given file. There is no useful interpretation of those system calls that doesn't involve an atomic replacement of the whole file. We don't need a separate system call: we already have the system calls. Nobody executing those system calls wants the dangerous interpretation of rename. At no time did an application developer sit down and think to himself, "I want to tell the kernel to perform an atomic rename, except when the system crashes. In that case, I want a zero-length file." Gods, no. Obviously, the application developer wanted to atomically replace the named file. Filesystems just need to honor the obvious intent of application developers.

Not reassuring

[Junta]

He presents three common cases for 'quickie' file modifications:
-Modify-in-place. Yes, this logically cannot be expected to leave the content intact in an unexpected interruption. You ask the OS to blow away data, then send it new data, there is a logical indeterminate state in the middle where doing things in the order you specified leaves you exposed.
-Write new file, use rename, using fsync to ensure a low exposure of data. This forces data to disk so it's coherent.
-Write new file and then use rename without fsync:
*This* he claims should easily be expected to corrupt the contents. I take issue with this. The fact that this occurs is because ext4 commits the rename out-of-order ahead of the data commit. I don't understand why the rename operation cannot also be delayed until after the data has been written out. I've seen several people ask 'I don't care that the change happens *now*, but I want the changes to occur in the order I specified', and thus far have seen Ts'o miss that point (intentionally or unintentionally). I have not read any explanation of why changing hardlinks should logically be an operation to jump ahead of pending data writeout. I could be missing something, but I'm not the only one with these questions.

fsync gives a relatively expensive guarantee above and beyond what people require to behave sanely. He says its inexpensive 'now' relative to the past. However, 'now' in this context only applies to ext4 users and thus the operation degrades other filesystem performance and fsync remains an expensive operation relative to not doing at all.

In terms of the general attitude of filesystems shrugging off data consistency so long as their indexes are intact, I find myself agreeing with Torvalds' comments on the debacle:
http://thread.gmane.org/gmane.linux.kernel/811167/focus=811700

Re:Ye

[TCM]

So you used the "riskier" fs for / where you don't actually need the features it provides and used the "more stable" fs where features could actually be useful because app/fs developers couldn't agree on semantics?

Only on Linux...

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.

Re:EXT4 is not broken?

[Jurily]

It's working exactly as designed. It's the applications that need fixing, no?

Does it matter whose fault it is when users are losing config files? It worked fine before, and now one of my basic expectations concerning Linux is broken: that no matter what happens short of hardware failure, I will not lose the files I already have. We're disappointed, and pointing fingers does not help.

Re:EXT4 is not broken?

[ivucica]

Didn't your mom teach you not to forcefully shut down any operating system with any file system? Just because it has measures to reduce the damage doesn't mean you can abuse it. So in this case, it is your fault.

And here I was going around all this time, feeling sorry for ext4 users who actually experienced system crashes due to bad graphics chip drivers or some other similar and silly problems. But no, it turns out that people who complain most are those who rely on operating system being able to resuscitate itself.

There's a reason why the filesystem syncs itself at the end of shutdown process, and why it is expected that you follow the process to the end. There's a reason why shutdown process exists in the first place. Throwing poor insults like "ext4 ranks with Windows 95" (perhaps you mean Win95's implementation of FAT?) doesn't help. Sure, it shouldn't lose stuff when the unexpected happens ... but you shouldn't rely and expect it will. Unexpected is just that -- unexpected -- and you'd better be prepared for it the next time your desktop falls over while it's turned off and your drive dies a horrible death. Because God, Buddha, Allah, Shiva or someone else will make sure that happens to you, if you've raised yourself to expect that FS will survive being constantly forcefully turned off.

kthxbye.

That is something I find peculiar...

[Junta]

When they went to journalling filesystems, by and large a simple mount operation turned into a mini-recovery operation, a psuedo-fsck if you will. This would even happen on read-only mounts, which to me violates expectations of no disk data being modified.

JFS had one 'quirk' that I think they got right, journal replay was an fsck-level event. A filesystem with a dirty journal could only be mounted read-only and the journal replay code was in fsck and had to be ran to enable remount read-write. There are numerous reasons why I stopped using JFS, but that is one point I kinda agreed with their quirkiness on.

Re:EXT4 is not broken?

[spitzak]

EXT4 is broken.

Posix requires that writing a file and then renaming it to a new location is an ordered atomic operation. Say file B already exists. You write file A, then close it, then rename (mv) it to B. Another program running at the same time opens B and reads it. It will get one of these two results, and NO OTHER RESULT:

1. It sees the old contents of B
2. It sees what was written to A.

EXT4 (before these patches) could result in the following result if your machine crashes and you start it again and look at B:

3. B is empty (also B is various partially-written versions of A, but empty most common).

Now it is true that Posix says that if the machine crashes, all bets are off. So yes EXT4 is being technically correct. But it would be equally technically correct if all the files on the disk were empty so this is pointless.

EXT4 promises to make crashes recoverable. This implies to me that after you recover from a crash, you will be left in a state allowed by POSIX. This means either you get the old contents of B or the new full contents of A, and EXT4 by allowing a different result is breaking it's design and promise.