Re: [RFC][PATCH] link.2: AT_ATOMIC_DATA and AT_ATOMIC_METADATA

Amir Goldstein
2019-06-03T06:17:33Z

> > Actually, one of my use cases is "atomic rename" of files with
> > no data (looking for atomicity w.r.t xattr and mtime), so this "atomic rename"
> > thread should not be interfering with other workloads at all.
>
> Which should already guaranteed because a) rename is supposed to be
> atomic, and b) metadata ordering requirements in journalled
> filesystems. If they lose xattrs across rename, there's something
> seriously wrong with the filesystem implementation. I'm really not
> sure what you think filesystems are actually doing with metadata
> across rename operations....
>

Dave,

We are going in circles so much that my head is spinning.
I don't blame anyone for having a hard time to keep up with the plot, because
it spans many threads and subjects, so let me re-iterate:

- I *do* know that rename provides me the needed "metadata barrier"
w.r.t. xattr on xfs/ext4 today.
- I *do* know the sync_file_range()+rename() callback provides the
"data barrier"
I need on xfs/ext4 today.
- I *do* use this internal fs knowledge in my applications
- I even fixed up sync_file_range() per your suggestion, so I won't need to use
the FIEMAP_FLAG_SYNC hack
- At attempt from CrashMonkey developers to document this behavior was
"shot down" for many justified reasons
- Without any documentation nor explicit API with a clean guarantee, users
cannot write efficient applications without being aware of the filesystem
underneath and follow that filesystem development to make sure behavior
has not changed
- The most recent proposal I have made in LSF, based on Jan's suggestion is
to change nothing in filesystem implementation, but use a new *explicit* verb
to communicate the expectation of the application, so that
filesystems are free
the change behavior in the future in the absence of the new verb

Once again, ATOMIC_METADATA is a noop in preset xfs/ext4.
ATOMIC_DATA is sync_file_range() in present xfs/ext4.
The APIs I *need* from the kernel *do* exist, but the filesystem developers
(except xfs) are not willing to document the guarantee that the existing
interfaces provide in the present.

[...]
> So, in the interests of /informed debate/, please implement what you
> want using batched AIO_FSYNC + rename/linkat completion callback and
> measure what it acheives. Then implement a sync_file_range/linkat
> thread pool that provides the same functionality to the application
> (i.e. writeback concurrency in userspace) and measure it. Then we
> can discuss what the relative overhead is with numbers and can
> perform analysis to determine what the cause of the performance
> differential actually is.
>

Fare enough.

> Neither of these things require kernel modifications, but you need
> to provide the evidence that existing APIs are insufficient.

APIs are sufficient if I know which filesystem I am running on.
btrfs needs a different set of syscalls to get the same thing done.

> Indeed, we now have the new async ioring stuff that can run async
> sync_file_range calls, so you probably need to benchmark replacing
> AIO_FSYNC with that interface as well. This new API likely does
> exactly what you want without the journal/device cache flush
> overhead of AIO_FSYNC....
>

Indeed, I am keeping a close watch on io_uring.

Thanks,
Amir.