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" the question 'are these exact bits irrevocably committed to the media?' is pretty much impossible to answer"

Well, kind of. fsync()'s job is only to ensure caches are written to durable storage. Its job is not to ensure the integrity of durable storage. Your idea of fsync()'s "Right Thing" is not quite correct, because these types of durable storage failures can happen outside of the context of a write -- so it's a bit silly to point a finger at fsync().

For example, you can write bytes to disk today and the drive might experience corruption on that sector next week.

"(and even then, I would not bet my life on it)"

All hardware fails, so of course you shouldn't bet your life on it. That's why we have both onsite and offsite backups. The durable media itself can always fail and lose data -- even if there aren't any writes occurring. Data loss can even occur on a powered off system.

Again, fsync()'s job is only to ensure the data is moved to durable storage. Solving the problem of data loss is done in other ways -- for example raid5 addresses the problem of durable storage failing by using parity and extra copies.

The real takeaway from the blog is perhaps that Raft as a protocol is inadequate, in the same way a raid1 mirror system is inadequate in terms of protecting from data corruption on durable storage (which of the two mirrored drives has the correct version of the data?). I'm frankly surprised that this situation is undetectable. It's a solved problem in other storage layers.

"I'll probably stick to SQLite write-ahead logs, at least I can wrap my mind around those"

Well, keep in mind: those have the same problem in the event of durable storage failure.

> Again, fsync()'s job is only to ensure the data is moved to durable storage.

The problem is that every layer in the stack lies. They say "yes it is definitely written to durable storage" when it is just in some cache layer and about to be written.

> it is just in some cache layer and about to be written

Definitely not. I get that sometimes we find things that do lie, but lying about this is a huge P0 bug and everyone with an interest in data storage knows to be on the lookout for such brokenness. E.g. such people do not buy SSD that lack some sort of power fail flush protection.

"The problem is that every layer in the stack lies. "

Not enterprise gear. Reliable storage does exist. I have tested many vendors myself, and gone through spec sheets under NDA (as mentioned above).

"They say "yes it is definitely written to durable storage" when it is just in some cache layer and about to be written."

Enterprise hardware contains batteries specifically designed so that caches can still be written out to durable storage in the event of power loss. Have you ever dealt with managing battery learning cycles on a Dell PERC?

> One thing that comes to mind when reading things like this is 'Distributed journaling for distributed systems', but since nobody seems to be doing that, it's probably a silly idea

That's exactly what Redpanda, BookKeeper, LogDevice, Pravega, and countless other proprietary systems at big tech companies do.

There is no way to know for sure that a Byzantine system is actually operating in a way that replicated copies of your data are actually safely written. Both due to the exact same issue of the drives themselves, and also that Byzantine system software is liable to have a variety of bugs and invalid states that will keep it operating as normal, even though the nodes are actually in a fault mode. (the problem is twofold: 1) the node and/or system not refusing writes when in a fault state, 2) the system not actually knowing that it's in a fault state) Even if you do all kinds of Jepsen simulation and mathematical proofs of the software (including the operating system!), you still can't trust the drives.

I think the only way to solve the problem is new storage firmware and hardware that is open and guarantees a write is done. I'm sure some companies may claim such functionality but we need an open source architecture and code to be sure.

In the meantime I think synchronous writes to multiple nodes is the safest option. Avoids complexity and bugs in fancy software, and the hardware is what it is.

> There is no way to know for sure that a Byzantine system is actually operating in a way that replicated copies of your data are actually safely written.

Isn't this how Bitcoin adds to the ledger though? Using a merkle tree and slowing things down significantly with those 6+ confirmations.

Technically Bitcoin can work without ever safely writing to disk. It's potentially all magical caches until the firmware decides to put bits to media.

That's actually a really good point. It isn't dependent on the hardware, it depends on the math.

That’s right. Raft uses sync writes which is what redpanda uses.

Complexity and industrial level reference impls are crucial

> In the meantime I think synchronous writes to multiple nodes is the safest option.

This is what is being proposed though, right? Like no one is saying just use fsync, it's to use fsync across multiple systems, no?

> Even in a relatively simple modern storage stack, say a single NVMe Flash device on a local PCIe controller, the question 'are these exact bits irrevocably committed to the media?' is pretty much impossible to answer without an NDA with the disk manufacturer and a hefty dose of proprietary IOCTLs (and even then, I would not bet my life on it).

The answer is easy in all cases: No.

Media can always fail without notice.

Media can fail, but the parent there is saying they can fail with byzantine faults.

Most of us don't protect against such faults. (Neither Redpanda nor Kafka do, but that's not the point of the OP; the OP's point is that Kafka is not protecting against non-byzantine faults in the disk.)

Media failures after the data is written seems out of scope.

Using fsync() to flush data to disk is, generally speaking, the wrong way to do things. fsync() is like using a piece of wood to hammer a nail into the ground. It'll work under very limited circumstances, but will fail horribly to do the heavy lifting required in other cases. fsync()'s semantics are complex and poorly defined, doubly so if multiple threads or processes are involved. Who gets the i/o error for a failed write? Nobody knows! fsync() is probably one of the worst APIs warts in Linux/Unix.

Developers competent in matters of storage generally use other methods / APIs like open()ing a file with O_SYNC and then checking the results of write() or pwrite() which will clearly identify when data is or is not successfully written to disk. If you want more performance, use async writes and it'll perform just fine. Seeing fsync() in code is a sign that the code has a bad smell and probably needs to be thoroughly reviewed.

There may be performance reasons to still use fsync with O_DIRECT instead of only O_SYNC.

https://news.ycombinator.com/item?id=15539828

> sing fsync() to flush data to disk is, generally speaking, the wrong way to do things.

I think this is an old "statement" due to the Linux ext4 sync issue from quite awhile ago. As far as I know it has been fixed. I doubt it is discouraged on the BSDs.

But, curious, what is the Right Thing to do ? There are cases where you want to be sure the data reaches the platter.

On a mini I use to program on, if a filename started with an '@' sign, that said all writes went directly to disk. Based upon that were were able to create a roll-forward recovery for our custom applications. So, as far as I know, fsync(2) should accomplish the same thing using the open descriptor.

I stand by my position. The problem with fsync() is the question of when and where do errors get reported. If code checks errors on completion of each write it becomes significantly easier to know what write has failed. The error reporting is the granularity of the write being performed.

In contrast, fsync() has to report errors for everything that has happened since the last fsync() call. If you are doing anything complicated like a database, which is virtually every single modern application, you need to have some idea of which writes have failed to figure out how to recover. fsync() gives you none of that. Different .

In kernel you get results for each i/o performed, so filesystems can make the appropriate decision about how to respond to a failure.

If you're building a high performance application, fsync() is a disaster. Unrelated writes will get bundled up when different threads call fsync() on the same file descriptor. Having multiple file descriptors open on the same file is silly and causes issues with applications that need to have many file descriptors open. The Linux kernel goes to great lengths to ensure that filesystems are high performance for writes on multiple threads.

fsync() sucks for anything more complex than "I have a single file to write out".

it tends to be true. we do things at the application tier to minimize these things.

for example, assume you have to write A, B, C ... in syscalls it would be

write( A ), flush() write( B ), flush() write( C ), flush()

so if you add a debounce of say 4ms you get

Write( A ), Write( B ), Write( C ), Flush()

and saved 2 flush()-es

This is common with protocol aware storage applications.

And I guess the idea is furthermore that just because you don't fsync after every write does not mean that you haven't fsync-ed before responding to the user request saying the data is stored durably. I assume that you do actually guarantee the flush before returning a success to the user.

exactly. you improve latency and throughput at the same time. is kinda cool. by delaying the reponses just a small bit you get huge benefits

Wrong. Use something like aio or io_uring to submit 4 asynchronous writes in a single system call and you'll get way better performance. The kernel has all kinds of infrastructure that tries to coalesce and batch things that the write()+fsync() syscalls make horribly inefficient, and in the modern world of really fast nvme drives, you want to make your calls into the device driver as efficient as possible. You'll burn far fewer CPU cycles by giving the kernel the whole set of i/os in one single go, burn less on synchronization. It really is better to avoid write() + fsync().

what makes you think that we haven't tested this? seastar's io engine is defaulted to io-uring... there are about 10 things here to comment on. on optimized kernels we disable block coalescing at the kernel level, second we tell the kernel to use fifo, etc. these low hanging fruit was already something we've done for a very very long time.

Yes, this is exactly how it works. It effectively batches the sync part of multiple user requests, so none of those user requests are ack-ed until the sync completes. It is a low-latency analogue to checkpointing storage write-backs to minimize the number of round-trips to the I/O syscalls.

Right. But that line of thinking gets you to a place where one is like “how does anything work” haha.

In general this was a response to Confluent attempting to dismiss fsync() as a neat trick rather than an actual safety problem and why when we benchmarked we showcase the numbers we did.

Which numbers? The ones on the blog that shows Redpanda significantly lagging behind open source Kafka?

https://jack-vanlightly.com/blog/2023/5/15/kafka-vs-redpanda...

this is a burner account created at the time this post was up.

> Even in a relatively simple modern storage stack, say a single NVMe Flash device on a local PCIe controller, the question 'are these exact bits irrevocably committed to the media?' is pretty much impossible to answer without an NDA with the disk manufacturer and a hefty dose of proprietary IOCTLs (and even then, I would not bet my life on it).

This would limit throughput quite a bit and might significantly shorten the lifespan of the disk, but you could put the disk on a separate power supply that the computer could control. Don't consider data as written until you've power cycled the disk and been able to read that data back. :-)

> you need to be absolutely, positively, 100% sure that `fsync()` has done The Right Thing would be: "Good luck with that!"

But it's safe to assume the right thing is not done if we haven't even called `fsync` to start with and just hope and pray that the dirty page flushing mechanism does it for us.