We say that rename() is atomic because you will either see file A or file B. There are no other possible states.
With a crash, writes might not be committed. You might get an earlier filesystem state.
And the whole point of TFA is that this is perfectly fine as long as the metadata writes aren't committed before the data writes. That is:
Precondition: File at path A exits with data X
1. Create file at path B
2. Write data Y to file B.
3. Close file B
4. Rename B to A
5. Crash
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As long as the contents in the file at path A are either X or Y, then we have achieve atomicity in the sense used in TFA. This is what we mean by rename acting "atomically across crashes"
Note that we only care about the file at path A; all of these are fine:
- File at path B exists and has data Y
- File at path B exists and has garbage data
- File at path B doesn't exist
- File at path B exists and is of size 0
Again, manipulating links has nothing to do with file data.
You can sync to durable storage at any point, yes. But you cannot do it in a semantically useful fashion.
"I don't think anyone is talking about situations where B is being actively written to while the rename happens."
Well, you specifically are ignoring this. I'm not.
Filesystems are free to generate extra syncs whenever they like. You could even have a filesystem sync every single write operation to durable storage - why not?
Given a link to a file at path A exists. The file contains data X. There may be other links at paths C, D and E.
1. Acquire a filesystem-global lock on link ops (link/unlink et al, but not read/write).
2. Create a link to X at path B
3. Remove the link at path A
4. Release the filesystem-global lock.
Note: The data in X is not relevant. The data in X may be undergoing active modification while the above 4 steps are performed. The data in X may be memory mapped and may change many times during the "atomic" rename() syscall.
The atomic properties of rename() are only vis a vis link/unlink semantics within a single filesystem. Other files can't be created or deleted while the rename() is in progress. File data however is under no such restriction -- and in fact it cannot be because the file may be memory-mapped and modified without any syscall interactions. There is no system call sequence point to gate reads and writes.
What you are describing is fundamentally incompatible with how the system actually works. What you describe cannot be made to work, ever. It is architecturally invalid at a fundamental level.
> File data however is under no such restriction -- and in fact it cannot be because the file may be memory-mapped and modified without any syscall interactions. There is no system call sequence point to gate reads and writes.
Note a complete lack of mmap() in the list of operations above; you are trying to trust what I am asking for into something that is impossible, when what I'm asking for is definitely possible.
[Edit]
I'm also well aware of how rename works; the ask is that the link is not committed before the data. This is possible to do at the expense of some performance, but possibly less performance then using existing commit primitives (e.g. fsync or fdatasync)
Because it is not an issue of atomic behavior but merely ordering there's no reason to modify the kernel or touch syscalls. It would be far more reliable to provide this in a library -- rename(3) -- which would then automatically provide the benefit on every posix compatible filesystem.
It would also allow for a use case of wanting to rename a file without forcing a full data sync -- which could be performance critical behavior in some scenarios.
To recap:
No filesystems provide atomic syncing of data during rename(). The words don't even make sense. Ext doesn't do it, zfs doesn't do it, xfs doesn't do it. Achieving this would require mechanisms that don't exist.
Some filesystems do guarantee that an fsync will occur alongside a rename. There is no specific guarantee as to what file state will be synced. These filesystems do this to mask the impact of folks mis-using the API, paying an efficiency cost as a result.
In ext4, for example, the sync does NOT occur within the same transaction as the rename. The sync is NOT atomic (it would be wild if this were the case, as noted above -- architecturally inconsistent and extremely non-performant)
It would be ideal for this to happen in libc rather than in the kernel.
Yes everyone in the discussion agrees on this fact
> ...but it is not atomic, merely ordered. You've switched from talking about one to talking about the other and they are not the same
If the operations we are talking about are ordered, then this provides the atomicity that TFA is asking for. Properly ordering operations is a very common way of making a series of steps atomic.
No, it doesn't. I think we're reaching the heart of the disagreement here - around what "atomic" means.
Atomic means indivisible. It means that other things cannot happen at the same time - that the individual (possibly ordered) steps of an operation cannot be observed while in progress.
This is why it's absolutely nonsense to talk about atomicity across a crash.
The fundamental design of unix files and memory preclude this. It can't be done.
> ...the operation we wish to be atomic is 'replacing the mapping path A->data X with path A->data Y'.
The ordering makes this operation atomic! The fact that it might leave another file around (if a crash happens) is okay.