Re: BUG #19369: Not documented that io_uring on kernel versions between 5.1 and below 5.6 does not work

>
> Sure Andres, I am working on a patch which emits a useful error message
> too.
>

Hi All,

I prepared a patch that implements an early detection of unsupported
io_uring operations during PostgreSQL startup, before any I/O is attempted.
The patch focuses on

1. Clear error message at startup instead of cryptic EINVAL during
queries.

2. Immediate failure with actionable hints (upgrade kernel or use
io_method=worker)

3. Prevents PostgreSQL from starting in a broken state

Before the patch:

1. PostgreSQL started successfully

2. Connection attempts failed with EINVAL errors

After patch:

1. PostgreSQL refuses to start

2. Clear error message that looks like:

"FATAL: kernel does not support required io_uring operations"
"DETAIL: The kernel supports io_uring but lacks one or more of the
required opcodes (IORING_OP_READ, IORING_OP_WRITE, IORING_OP_READV,
IORING_OP_WRITEV). This typically occurs on Linux kernels older than 5.6."

"HINT: Either upgrade your kernel to version 5.6 or newer, or

use io_method=worker"

Modified files in the Patch:

1. configure.ac: Added io_uring_opcode_supported to AC_CHECK_FUNCS

2. meson.build: Added corresponding function check for Meson build

3. src/backend/storage/aio/method_io_uring.c: Added
is_uring_read_write_unsupported() function, that is integrated into
pgaio_uring_init() and reports clear error with details and hints.

I tested the patch on Ubuntu server with Linux kernel 5.4.0-216-generic,
and when io_uring is enabled I see that postgres doesn’t start (expected
behavior).

The existing error handling for kernels < 5.1 (ENOSYS) is preserved.

Regards,
Surya Poondla