POSIX is also so old and essential that it's hard to imagine an alternative.
The kind of model I prefer is something based on atomicity. Most applications can get by with file-level atomicity--make whole file read/writes atomic with a copy-on-write model, and you can eliminate whole classes of filesystem bugs pretty quickly. (Note that something like writeFileAtomic is already a common primitive in many high-level filesystem APIs, and it's something that's already easily buildable with regular POSIX APIs). For cases like logging, you can extend the model slightly with atomic appends, where the only kind of write allowed is to atomically append a chunk of data to the file (so readers can only possibly either see no new data or the entire chunk of data at once).
I'm less knowledgeable about the way DBs interact with the filesystem, but there the solution is probably ditching the concept of the file stream entirely and just treating files as a sparse map of offsets to blocks, which can be atomically updated. (My understanding is that DBs basically do this already, except that "atomically updated" is difficult with the current APIs).
int fd = open(".config", O_RDWR | O_CREAT | O_SYNC_ON_CLOSE, 0o666);
// effects of calls to write(2)/etc. are invisible through any other file description
// until the close(2) is called on all descriptors to this file description.
close(fd);
What happens when a lot of data is written and exceeds the dirty threshold?
Database developers don’t want the complexity or poor performance of posix. It’s wild to me that we still don’t have any alternative to fsync in Linux that can act as a barrier without also flushing caches at the same time.
You could probably abuse Force Unit Access to make it work by marking all IOs as Force Unit Access, but a number of buggy devices do not implement FUA properly, which defeats the purpose of using it. That would be why Microsoft disabled the NTFS feature that uses FUA on commodity hardware:
https://learn.microsoft.com/en-us/windows/win32/fileio/deplo...
What you seem to want is FreeBSD’s UFS2 Softupdates that uses force unit access to avoid the need for flushes for metadata updates. It has the downside that it is unreliable on hardware that does not implement FUA properly. Also, UFS2 softupdates does not actually implement do anything to protect data when fsync(2) is called if this mailing list email is accurate:
https://lists.freebsd.org/pipermail/freebsd-fs/2011-November...
As pjd said:
> Synchronous writes (or BIO_FLUSH) are needed to handle O_SYNC/fsync(2) properly, which UFS currently doesn't care about.
That said, avoiding flushes for a fsync(2) would require doing FUA on all IOs. Presumably, this is not done because it would make all requests take longer all the time, raising queue depths and causing things to have to wait for queue limits more often, killing performance. Raising the OS queue depth to compensate would not work since SATA has a maximum queue depth of 32, although it might work for NVMe where the maximum queue depth is 65536, if keeping track of an increased number of inflight IOs does not cause additional issues at the storage devices (such as IOs that never complete as long as the device is kept busy because the device will keep reordering them to the end of the queue).
Using FUA only on metadata as is done in UFS2 soft updates improves performance by eliminating the need for journalling in all cases but the case of space usage, which still needs journalling (or fsck after power loss if you choose to forgo it).
- This has not already happened with a lot of the old C standard library. The only function that has ever been removed from the C standard library, to my knowledge, is gets(). In particular, strcpy() has not been removed. Current popular compilers still support gets() with the right options, so it hasn't been removed from the actual library, just the standard.
- strncpy() is not a suitable replacement for strcpy(), certainly not a safer one. It can produce strings missing the terminating null, and it can be slower by orders of magnitude. This has been true since it was introduced in the 01970s. Nearly every call to strncpy() is a bug, and in many cases an exploitable security hole. You are propagating dangerous misinformation. (This is a sign of how difficult it is to make these transitions.)
You also seem to imply that Linux cannot add system calls that are not specified in POSIX, but of course it can and does; openat() and the other 12 related functions, epoll_*(), io_uring_*(), futex_*(), kexec_load(), add_key(), and many others are Linux-specific. The reason barrier() hasn't been added is evidently that the kernel developers haven't been convinced it's worthwhile in the 15+ years since it was proposed, not that POSIX ties their hands.
The nearest equivalents in C for the kind of "staged transition" you are proposing might be things like the 16-bit near/far/huge qualifiers and the Win16 and pre-X MacOS programming models. In each of these cases, a large body of pre-existing software was essentially abandoned and replaced by newly written software.
I don’t understand the reticence of kernel developers to implement a barrier syscall. I know they could do it. And as this article points out, it would dramatically improve database performance for databases which make use of it. Why hasn’t it happened?
Another commenter says NVMe doesn’t support it natively but I bet hardware vendors would add hardware support if Linux supported it and adding barrier support to their hardware would measurably improve the performance of their devices.
Unless you mean memcpy(), there is in fact no safer alternative function in the C standard for strcpy(); software has not largely moved to not using strcpy() (plenty of new C code uses it); and most validators and sanitizers do not emit warnings for strcpy(). There is a more extensive explanation of this at https://software.codidact.com/posts/281518. GCC has warnings for some uses of strcpy(), but only those that can be statically guaranteed to be incorrect: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
Newer, safer alternatives to strcpy() include strlcpy() and strscpy() (see https://lwn.net/Articles/659818/), neither of which is in Standard C yet. Presumably OpenBSD has some sort of validator that recommends replacing strcpy() with strlcpy(), which is licensed such that you can bundle it with your program. Visual C++ will invite you to replace your strcpy() calls with the nonstandard Microsoft extension strcpy_s(), thus making your code nonportable and, as it happens, also buggy. An incompatible version of strcpy_s() has been added as an optional annex to the C11 standard. https://nullprogram.com/blog/2021/07/30/ gives extensive details, summarized as "there are no correct or practical implementations". The Linux kernel's checkpatch.pl will invite you to replace calls to strcpy() with calls to the nonstandard Linux/BSD extension strscpy(), but it's a kernel-specific linter.
So there are not literally zero validators and sanitizers that will warn on all uses of strcpy() in C, but most of them don't.
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I don't know enough about the barrier()/osync() proposal to know why it hasn't been adopted, and obviously neither do you, since you can't know anything significant about Linux kernel internals if you think that C has methods or that strncpy() is a safer alternative to strcpy().
But I can speculate! I think we can exclude the following possibilities:
- That the paper, which I haven't read much of, just went unnoticed and nobody thought of the barrier() idea again. Luu points out that it's a sort of obvious idea for kernel developers; Chidambaram et al. ("Optimistic Crash Consistency") weren't even the first ones to propose it (and it wasn't even the main topic of their paper); and their paper has been cited in hundreds of other papers, largely in systems software research on SSDs: https://scholar.google.com/scholar?cites=1238063331053768604...
- That it's a good idea in theory, but implementing even a research prototype is too much work. Chidambaram et al.'s code is available at https://github.com/utsaslab/optfs, and it is of course GPLv2, so that work is already done for you. You can download a VM image from https://research.cs.wisc.edu/adsl/Software/optfs/ for testing.
- That authors of databases don't care about performance. The authors of SQLite, which is what Chidambaram et al. used in their paper, dedicate a lot of effort to continuously improving its performance: https://www.sqlite.org/cpu.html and it's also a major consideration for MariaDB and PostgreSQL.
- That there's an existing production-ready implementation that Linus is just rejecting because he's stubborn. If that were true, you'd see an active community around the OptFS patch, Red Hat applying it to their kernels (as they do with so many other non-Linus-accepted patches), etc.
- That it relies on asynchronous barrier support in the hardware interface, as the other commenter suggested. It doesn't.
So what does that leave?
Maybe the paper was wrong, which seems unlikely, or applicable only to niche cases. You should be able to build and run their benchmarks.
Maybe it was right at the time on spinning rust ("a Hitachi DeskStar 7K1000.B 1 TB drive") but wrong on SSDs, whose "seek time" is two to three orders of magnitude faster.
In particular, maybe it uses too much CPU.
Maybe it was right then and is still right but the interface has other drawbacks, for example being more bug-prone, which also seems unlikely, or undesirably constrains the architecture of other aspects of the kernel, such as the filesystem, in order to work well enough. (You could implement osync() as a filesystem-wide fsync() as a fallback, so this would just reduce the benefits, not increase the costs.)
Maybe it's obviously the right thing to do but nobody cares enough about it to step up and take responsibility for bringing the new system call up to Linus's standards and committing to maintain it over time.
If it was really a big win for database performance, you'd think one of the developers of MariaDB, PostgreSQL, or SQLite would have offered, or maybe one of the financial sponsors of the paper, which included Facebook and EMC. Luu doesn't say Twitter used the OptFS patch when he was on the Linux kernel team there; perhaps they used it secretly, but more likely they didn't find its advantages compelling enough to use.
Out of all these unlikely cases, my best guess is either "applicable only to niche cases", "wrong on SSDs", or "undesirably constrains filesystem implementation".
As a note on tone, some people may find it offputting when you speak so authoritatively about things you don't know anything about.
This is an overly pedantic, ungenerous interpretation of what I wrote.
First, fine - you can argue that C has functions, not methods. But eh.
Second, for all practical purposes, C on Linux does have a standard library. It’s just - as you mentioned - not quite the same on every platform. We wouldn’t be talking about strcpy if C had no standard library equivalent.
Third, thankyou for the suggestion that there are even better examples than strcpy -> strncpy that I could have used to make my point more strongly. I should have chosen sprintf, gets or scanf.
I’ve been out of the game of writing C professionally for 15 years or so. I know a whole lot more about C than most. But memories fade with time. Thanks for the corrections. Likewise, no need to get snarky with them.
The reason it hadn't yet been supported btw. is that they explicitly wanted to allow fully parallel processing of commands in a queue, at least for submissions that concurrently exist in the command queue. In practice I don't see why this would have to be enforced to such an extend, as the only reason for out-of-order processing I can think of is that the auxiliary data of a command is physically located in host memory and the DMA reads across PCIe from the NVMe controller to the host memory happen to complete out-of-order for host DRAM controller/pattern reasons. Thus it might be something you'd not want to turn on without using controller memory buffer (where you can mmap some of the DRAM on the NVMe device into host memory, write your full-detail commands directly to this across the PCIe, and keep the NVMe controller from having to first send a read request across PCIe in response to you ringing it's doorbell: instead it can directly read from it's local DRAM when you ring the doorbell).
That said, IO barriers in storage are typically synonymous with flushes. For example, the ext4 nobarrier mount option disables flushes.
I have many files that are several GB. Are you sure this is a good idea? What if my application only requires best effort?
> eliminate whole classes of filesystem bugs pretty quickly.
Block level deduplication is notoriously difficult.
> where the only kind of write allowed is to atomically append a chunk of data to the file
Which sounds good until you think about the complications involved in block oriented storage medium. You're stuck with RMW whether you think you're strictly appending or not.
But even then, doing atomic writes of multi gigabyte files doesn’t sound that hard to implement efficiently. Just write to disk first and update the metadata atomically at the end. Or whenever you choose to as a programmer.
The downside is that, when overwriting, you’ll need enough free space to store both the old and new versions of your data. But I think that’s usually a good trade off.
It would allow all sorts of useful programs to be written easily - like an atomic mode for apt, where packages either get installed or not installed. But they can’t be half installed.
Databases implemented atomic transactions in the 70s. Let’s stop pretending like this is an unsolvable CS problem. Its not.
And they have deadlocks as a result, which there is no good easy solution to (generally we work around by having only one program access a given database at a time, and even that is not 100% reliable).
It works great in practice, even with a lot of concurrent clients. (iCloud is all built on foundationdb).
Hold & lock is what causes deadlocks. I agree with you - that would be a bad way to implement filesystem transactions. But we have a lot of other options.
If you want atomic updates with APT, you could look into doing prestaged updates on ZFS. It should be possible to retrofit it into APT. Have it update a clone of the filesystem and create a new boot environment after it is done. The boot environment either is created or is not created. Then reboot into the updated OS and you can promote the clone and delete the old boot environment afterward. OpenSolaris had this capability over a decade ago.
Provided the underlying VFS has implemented them. They may not. Hence the point in the article that some developers only choose to support 'ext4' and nothing else.
> you’ll need enough free space to store both the old and new versions of your data.
The sacrifice is increased write wear on solid state devices.
> It would allow all sorts of useful programs to be written easily
Sure. As long as you don't need multiple processes to access the same file simultaneously. I think the article misses this point, too, in that, every FS on a multi user system is effectively a "distributed system." It's not distributed for _redundancy_ but it doesn't eliminate the attendant challenges.
As for wear on SSDs, I don’t think it would increase wear. You’re writing the same number of sectors on the drive. A 2gb write would still write 2gb (+ negligible metadata overhead). Why would the drive wear out faster in this scheme?
And I think it would work way better with multiple processes than the existing system. Right now the semantics when multiple processes edit the same file at once are somewhat undefined. With this approach, files would have database like semantics where any reader would either see the state before a write or the state after. It’s much cleaner - since it would become impossible for skewed reads or writes to corrupt a shared file.
Would you argue against the existence of database transactions? Of course not. Nobody does. They’re a great idea, and they’re way easier to reason about and use correctly compared to the POSIX filesystem api. I’m saying we should have the same integrity guarantees on the filesystem. I think if we had those guarantees already, you’d agree too.
https://help.dropbox.com/installs/system-requirements
They say ecryptfs is only supported when it is backed by ext4, which is a bit strange. I wonder if that is documented just to be able to close support cases when ecryptfs is used on top of a filesystem that is missing extended attribute support and their actual code does not actually check what is below ecryptfs. Usually the application above would not know what is below ecryptfs, so they would need to go out of their way to check this in order to enforce that. I do not use Dropbox, so someone else would need to test to see if they actually enforce that if curious enough.
https://github.com/openzfs/zfs/blob/34205715e1544d343f9a6414...
Writes on ZFS cease to be atomic around approximately 32MB in size if I read the code correctly.