Why Unix needs a standard way to deal with the file durability problem

https://utcc.utoronto.ca/~cks/space/blog/unix/WhyFileSyncStandardNeeded

by zdw • 10 years ago

123 36 10 years ago

utcc.utoronto.ca

ape4• 10 years ago

No this isn't the same article that was posted the other day. Is a response to comments.

haberman• 10 years ago

IMO the way to tackle a problem like this is:

1. Figure out the right primitive(s) that applications need to provide both correctness and performance. Make these as simple as possible.

2. Make a portable library that implements the simple primitives by doing whatever is necessary on each platform to provide the API's guarantees. This will likely require a lot of research/expertise on each platform. The result will be an ugly mess of #ifdef, but using the library will not be ugly.

3. If implementing the library is impossible or unnecessarily slow on some platform, lobby the maintainers of that platform to provide something better. If they fundamentally disagree with the abstraction, put it on them to counter-propose a better one.

4. (Optional) lobby to have the primitive become part of the C standard library or POSIX to shift the maintenance burden to the platform owners.

geocar• 10 years ago

Okay, here's a stab at it:

• SIGIOFIN works like SIGIO except it fires when the IO operation completes (is on the disk; has left the network card, etc).

I should be able to use signalfd() to collect these. Let me use TCP_DEFER_ACCEPT on regular file descriptors -- basically let me make sure there are n bytes before being woken up on a fifo, and n events in my signalfd() before waking up.

Here's another idea:

• all() returns a file descriptor that is initially readable.

• on(POLLIN,f,d) while poll(f,POLLIN) would block, d is unreadable

• POLLSYNC added to poll(), etc which is flagged when all previous writes to the file descriptor have reached the disk (has left the network card, etc). To get multiple checkpoints use dup().

I'm not sure which one I'd like better. If we didn't have to make it "primitive", we could simply copy what windows does[1], since they figured this out years ago.

What do you think?

[1] https://msdn.microsoft.com/en-gb/library/windows/desktop/aa3...

gue5t3• 10 years ago

I advise skipping the signal nonsense and making the interface purely fd-based. The only "advantage" of signals over fd readiness is that signals can interrupt syscalls that don't expect them, and this can be emulated without undue performance loss using asynchronous requests and epoll-style architectures. Signals cause a lot of pain in concurrent (e.g. coroutines and reentrancy) and multithreaded contexts. Plus, including data about which IO request finished and whether it succeeded or failed is more complicated with signals than simply including it in the event structs read from an fd.

Another problem is that this doesn't really address the synchronous assumptions of filesystem (rather than file content) operations: applications expect that they can "mkdir(...); chdir(...); open(...);" and the filesystem doesn't know if the application would work properly with those operations reordered (it may or may not depending on the values of their arguments). Applications need to be more explicit with the transactional nature of what they're requesting from filesystems. Right now the filesystem tries to guess at the transaction structure based on fsync/fdatasync hints, and can't do as good a job scheduling as a result.

zAy0LfpBZLC8mAC• 10 years ago

(1) This doesn't address ordering guarantees independently from durability guarantees (there are cases where you need to make sure writes don't get reordered, but durability right now is not required).

(2) Signals are just a terrible interface, especially because they are process (or at least thread) global, so they are incredibly hard to use in libraries.

daurnimator3• 10 years ago

Why not just a fd flag that makes it so `write` doesn't return until the write has hit the disk?

Oh wait... isn't that O_SYNC/O_DSYNC? In which case, the problem is really using O_SYNC file descriptors asynchronously.

Idea:

Return EAGAIN if you try to write and the last write hasn't finished yet. + the file descriptor returns POLLOUT only when no writes are in flight.

This unfortunately means you can't have multiple writes in flight.

-----------------------

Wait, this is all too familiar....

I think this is solved by the linux kernel's existing async IO system.

Relatively unknown is that you can pass an eventfd file descriptor to async io operations (see https://gist.github.com/daurnimator/778c36c2de32f8882d0d for an example). With IOCB_CMD_FSYNC + IOCB_CMD_FDSYNC we should have everything we need?

I wonder if O_SYNC works with IOCB_CMD_PWRITE?

The kernel aio interface is not wrapped by libc, only works on some file systems, has some odd-er alignment+size requirements and only supported by linux... though posix aio may be similar enough, I'd need to check.

------------------------

I did just find http://lkml.iu.edu/hypermail/linux/kernel/1601.1/02634.html which seems like an interesting development in this area.

tome• 10 years ago

When I read the first article I did wonder why there couldn't be a standard API for this. Thanks for asking the question.

gue5t3• 10 years ago

I'm so glad this is being discussed. The intricate and weak durability guarantees, the unaddressed dangers of global mutable state (without reasonable locking capabilities outside of "only ever write something so small the OS wouldn't ever decide to break it up"), and the inability to properly deal with files and the filesystem asynchronously are gaping holes in POSIX (and Linux) semantics.

Maybe we can actually get a powerful asynchronous IO API that allows the OS to schedule disk requests effectively and applications to avoid blocking in situations where they don't have to, while providing more guarantees about data durability and concurrency. It's undeniably possible, but it will require a level of coordination between parties (standardization and work from kernel hackers in Linux and other OSes, userspace libraries to expose new kernel functionality, applications switching over to use the libs) that I hadn't expected to see come together. Maybe these discussions will provide some impetus to get people working on all sides of the issue.

Anyone who hasn't read them should see the comments on previous discussion of this topic, which demonstrate how hairy this area is right now: https://news.ycombinator.com/item?id=11511269

ipozgaj• 10 years ago

Even if API gave you strong durability guarantees, it still wouldn't mean much. Disk caches, big enterprise SAN attached storages etc, they can also "cheat", saying they flushed the cache while they actually didn't.

yason• 10 years ago

The API allows blaming the right cog in the machinery. Now everyone gets a Get Out Of Jail Free card because they can blame the kernel, filesystem drivers, userspace libraries, application developers, disk controllers, and whatnot, and thus nothing forces a single direction in that cyclic graph of blame.

zzzcpan• 10 years ago

I still don't see why this is such a big deal. Let's say everyone agrees on a standard and guarantees to make fsync() send data to a disk. Does this guarantee even worth anything? Will every disk suddenly disable write cache on itself by default? Who is going to sacrifice performance and make it a default? Even if somebody is going to do it, will disks suddenly become bug-free, 100% reliable and indestructible?

fulafel2• 10 years ago

fsync works fine with on disk write caches (if the OS so chooses, ie not OS X). It's all specified in SATA and SCSI. Yes you can have bugs and lying devices, but reputable stuff is generally fine.

mtanski4• 10 years ago

Many consumer drives tend to omit this feature (or lie about it) because all anybody looks at is size and iops. Nobody sells more consumer drives by advertising that the writeback cache on the drive respects flush commands (and barriers).

wyc• 10 years ago

Here's a relevant passage from the essay "Worse Is Better" by Richard P. Gabriel:

http://dreamsongs.com/WIB.html

"""

Two famous people, one from MIT and another from Berkeley (but working on Unix) once met to discuss operating system issues. The person from MIT was knowledgeable about ITS (the MIT AI Lab operating system) and had been reading the Unix sources. He was interested in how Unix solved the PC loser-ing problem. The PC loser-ing problem occurs when a user program invokes a system routine to perform a lengthy operation that might have significant state, such as IO buffers. If an interrupt occurs during the operation, the state of the user program must be saved. Because the invocation of the system routine is usually a single instruction, the PC of the user program does not adequately capture the state of the process. The system routine must either back out or press forward. The right thing is to back out and restore the user program PC to the instruction that invoked the system routine so that resumption of the user program after the interrupt, for example, re-enters the system routine. It is called PC loser-ing because the PC is being coerced into loser mode, where loser is the affectionate name for user at MIT.

The MIT guy did not see any code that handled this case and asked the New Jersey guy how the problem was handled. The New Jersey guy said that the Unix folks were aware of the problem, but the solution was for the system routine to always finish, but sometimes an error code would be returned that signaled that the system routine had failed to complete its action. A correct user program, then, had to check the error code to determine whether to simply try the system routine again. The MIT guy did not like this solution because it was not the right thing.

"""

ianlevesque• 10 years ago

Further reading http://blog.reverberate.org/2011/04/eintr-and-pc-loser-ing-i...

infinity0• 10 years ago

The quote continues, "The New Jersey guy said that the Unix solution was right because the design philosophy of Unix was simplicity and that the right thing was too complex. Besides, programmers could easily insert this extra test and loop. The MIT guy pointed out that the implementation was simple but the interface to the functionality was complex. The New Jersey guy said that the right tradeoff has been selected in Unix-namely, implementation simplicity was more important than interface simplicity. "

As I see it, too many people abuse / misunderstand "worse is better". "Worse" might be better than "perfect" in some cases, but "perfect" is better than "crap", and "trying to be worse" usually results in "crap" whereas "trying to be perfect" usually results in "worse".

Even in the above example, the MIT guy pointed out that the simple implementation results in a complex interface, and undoubtedly much much more time was wasted in the end.

jstimpfle2• 10 years ago

So I never dug deep into the POSIX but I still miss this question answered before I accept the blog post's premise:

What's wrong with the standard procedure

    1. open / create file
    2. write to file
    3. sync file
    4. possibly rename file to final location
    5. sync parent directory of file

Which compliant file systems don't guarantee that this is durable (notwithstanding disks that tell lies)?

Maybe POSIX doesn't specify this exact sequence explicitly (don't know), but this is the folklore which you find everywhere, and it's just what you arrive at naturally when you consider the object model on which the API is built (where an object is an "open file", i.e. what fds reference). (I mentioned this: it's basically the model that git also adopts, except that git objects are immutable).

daurnimator3• 10 years ago

The issue is that most programs do not do step 3: e.g. fsync is not called by C's stdio.

Which means that the `rename` might get reordered before the `write`. Which means data loss. yay!

jstimpfle2• 10 years ago

The discussion hasn't been about stdio so far.

Btw. my fflush(3) manpage does mention that fsync(2) is required to ensure data reaches the disk. Of course it's not nice that we have to drop abstractions (and portability) to go the safe way. Let's just say stdio is a bit simplistic.

Btw. many file systems don't reorder write and rename for exactly this reason. But at least ext2 and btrfs seem to do it.

thatcks• 10 years ago

What is wrong with the standard procedure is that once upon a time the standard procedure that everyone knew and coded to was steps 1 through 4, without step 5. Then Linux ext4 people decided that step 5 was necessary, or rather that they could optimize ext4 to delay and lose some writes if you left out step 5. Which lots of people did, and so some people unexpectedly lost data and were unhappy.

If we have only a standard procedure and not an actual agreed on standard (whether or not it's a formal one), we are forever vulnerable to the next clever filesystem developer who decides that obviously they can get away with adding a new step 6, or 7, or 8 in some situations, under some interpretation of the POSIX or Single Unix Specification wording, or the Linux manpages, or whatever.

(I'm the author of the linked-to blog entry.)

Drdrdrq• 10 years ago

While OP makes good points, I think there is no way to guarantee data persistence on a single node. Especially with the SSDs.

ubernostrum• 10 years ago

OP is not asking for something that will guarantee a complete, safe write on all systems under all conditions in all logically-possible universes.

OP is asking for something that, if it loses data due to the way the filesystem or operating system behaves, would be treated as a bug in the filesystem or OS, rather than a "lol, this idiot didn't even do (set of syscalls unique to the particular FS/OS combination), not our fault it didn't actually write after it said it would".

jeffdavis• 10 years ago

The UNIX filesystem should not be used for applications, in my opinion. And for the underlying technologies, like databases, object stores, etc, it's the only option (but not a great one).

Even for something simple like photos, it's not good:

* durability problems

* hierarchy doesn't work out well (browsing by dates vs people vs event types can't all work in the same hierarchy)

* you want storage on and accessibility from more than one machine

* something like content-addressed storage probably makes sense to avoid unnecessary duplication

Then, for underlying tech like databases it's not great either. Postgres used to initialize the data directory without syncing it. That makes sense for production, because who is going to put important data in it and crash within a few minutes? But it caused problems with testing, so I fixed it. To do such a trivial thing: write out a bunch of files and sync the whole directory, it was very non trivial. Syncing the files individually was very slow (because I have no clue what the most convenient order is for the OS to sync the files). I had to use the linux-specific sync_file_range() to get decent performance, and then it was still slow so we needed a way to optionally turn it off (e.g. for tests where it's not important). Horrible. It would have been much better to just say: do all of these things and sync them in the most convenient order, then tell me when you are finished. Kind of like a log based system.

cyphar• 10 years ago

You can fake multiple hierarchies with hard links. The only problem is you can't delete every hardlink easily.

mtanski4• 10 years ago

Your app has to define it's own cascading delete strategy.

Animats• 10 years ago

I wrote about this yesterday, in yesterday's topic.[1]

Some of the ideas I presented there are from UCLA Locus, circa 1980.[2] UCLA Locus was a distributed UNIX, and an improvement on most of its successors. Multiple machines on a network could not only share files, but you could run a program on a different machine than its files. The machines didn't even have to have the same type of CPU.

Overwriting a file resulted in creating a new file which initially shared all its blocks with the old one. Updating was a copy-on-write type operation. The new file became visible to other readers when closed, or when the writer called "commit()". If the writer crashed, or a network connection was lost, the file reverted. There was also a "revert()" call. Readers thus always saw a consistent version of the file.

Locus also supported replicated files, where two nodes had copies of the same file, but the file was treated as being a single file. If changed, all nodes saw the change.

This was before there were databases for UNIX, and so they gave the file system database-like semantics. It was a good idea that didn't catch on. The technology was sold several times (the sale to SCO was the killer) and eventually open-sourced.[3] It hasn't been updated since about 2010, though.

[1] https://news.ycombinator.com/item?id=11511269 [2] https://en.wikipedia.org/wiki/LOCUS_%28operating_system%29 [3] http://openssi.org/cgi-bin/view?page=openssi.html

marvy2• 10 years ago

I said this in the previous thread and I'll say it here too: Rethink the Sync is an attempt to solve this problem from ten years ago.

https://www.usenix.org/legacy/events/osdi06/tech/nightingale...

Basically (read my comment in the other thread for a bit more info), you get file durability and good performance without ever having to type the word "sync", all with the standard POSIX interface.

jordonwii• 10 years ago

That paper was the first thing I thought of when I read this, too. It really is a fascinating solution to this problem. Any ideas why external synchrony didn't catch on?

I suppose one could argue that the implementation would add a decent amount of conceptual overhead to the kernel, for comparatively little reward (application authors can fsync now, etc.).

marvy2• 10 years ago

I wonder if the Linux folks are even aware of this paper. Maybe they're not?

Why Unix needs a standard way to deal with the file durability problem

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