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

Hi Evsi,

I created a new Ubuntu server and am able to reproduce the same behavior
you reported for Postgres 18.1 on aarch64 architecture.

surya(at)surya:~/pgdata18/log$ uname -r
5.4.0-216-generic
surya(at)surya:~/pgdata18/log$ uname -a
Linux surya 5.4.0-216-generic #236-Ubuntu SMP Fri Apr 11 19:55:34 UTC 2025
aarch64 aarch64 aarch64 GNU/Linux

I did some testing and see the below
In the postgresql.conf if I put
io_method = 'worker' or io_method = 'sync', postgres is able to restart
with no errors.
But if io_method = 'io_uring' then I see the below on my console

surya(at)surya:~/postgresql-18.1$ vim ~/pgdata18/postgresql.conf
surya(at)surya:~/postgresql-18.1$ ~/pg18/bin/pg_ctl -D ~/pgdata18 restart
waiting for server to shut down....2026-01-08 04:06:07.081 UTC [44973]
DEBUG: logger shutting down
2026-01-08 04:06:07.081 UTC [44973] DEBUG: shmem_exit(0): 0
before_shmem_exit callbacks to make
2026-01-08 04:06:07.081 UTC [44973] DEBUG: shmem_exit(0): 0 on_shmem_exit
callbacks to make
2026-01-08 04:06:07.081 UTC [44973] DEBUG: proc_exit(0): 0 callbacks to
make
2026-01-08 04:06:07.081 UTC [44973] DEBUG: exit(0)
2026-01-08 04:06:07.081 UTC [44973] DEBUG: shmem_exit(-1): 0
before_shmem_exit callbacks to make
2026-01-08 04:06:07.081 UTC [44973] DEBUG: shmem_exit(-1): 0 on_shmem_exit
callbacks to make
2026-01-08 04:06:07.081 UTC [44973] DEBUG: proc_exit(-1): 0 callbacks to
make
done
server stopped
waiting for server to start....2026-01-08 04:06:07.172 GMT [44988] LOG:
invalid value for parameter "io_method": "io_uring"
2026-01-08 04:06:07.172 GMT [44988] HINT: Available values: sync, worker.
2026-01-08 04:06:07.172 UTC [44988] FATAL: configuration file
"/home/surya/pgdata18/postgresql.conf" contains errors
2026-01-08 04:06:07.172 UTC [44988] DEBUG: shmem_exit(1): 0
before_shmem_exit callbacks to make
2026-01-08 04:06:07.172 UTC [44988] DEBUG: shmem_exit(1): 0 on_shmem_exit
callbacks to make
2026-01-08 04:06:07.172 UTC [44988] DEBUG: proc_exit(1): 0 callbacks to
make
2026-01-08 04:06:07.172 UTC [44988] DEBUG: exit(1)
stopped waiting
pg_ctl: could not start server
Examine the log output.

From the above we can see that the stderr says that "*invalid value for
parameter "io_method": "io_uring"*"

And in the logs I see similar thing like you reported
2026-01-08 04:05:31.438 UTC [44972] DEBUG: starting background worker
process "logical replication launcher"
2026-01-08 04:05:31.438 UTC [44981] DEBUG: autovacuum launcher started
2026-01-08 04:05:31.438 UTC [44981] DEBUG: InitPostgres
2026-01-08 04:05:31.438 UTC [44982] DEBUG: logical replication launcher
started
2026-01-08 04:05:31.438 UTC [44982] DEBUG: InitPostgres
2026-01-08 04:05:31.438 UTC [44981] DEBUG: io 6400 |op invalid|target
invalid|state HANDED_OUT : adding cb #1, id
2/aio_shared_buffer_readv_cb
2026-01-08 04:05:31.438 UTC [44981] DEBUG: io 6400 |op invalid|target
smgr|state HANDED_OUT : adding cb #2, id 1/aio_md_readv_cb
2026-01-08 04:05:31.438 UTC [44981] DEBUG: io 6400 |op readv|target
smgr|state DEFINED : calling cb #1
2/aio_shared_buffer_readv_cb->stage(0)
2026-01-08 04:05:31.438 UTC [44981] DEBUG: io 6400 |op readv|target
smgr|state STAGED : staged (synchronous: 0, in_batch: 1)
2026-01-08 04:05:31.439 UTC [44974] DEBUG: io 6400 |op readv|target
smgr|state COMPLETED_IO : after shared completion: distilled result:
(status OK, id 0, error_data: 0, result 1), raw_result: 8192
2026-01-08 04:05:31.439 UTC [44981] DEBUG: io 6400 |op readv|target
smgr|state COMPLETED_SHARED: after local completion: result: (status OK, id
0, error_data 0, result 1), raw_result: 8192

I am working on a patch and will fix the inconsistencies in documentation
about io_uring.

Best,
Surya Poondla

On Wed, Jan 7, 2026 at 7:54 PM PG Bug reporting form <noreply(at)postgresql(dot)org>
wrote:

> The following bug has been logged on the website:
>
> Bug reference: 19369
> Logged by: Evan Si
> Email address: evsi(at)amazon(dot)com
> PostgreSQL version: 18.1
> Operating system: Amazon Linux 2
> Description:
>
> Hello,
>
> The documentation doesn't explicitly state a minimum kernel version for
> io_uring, but in src/backend/storage/aio/README.md, it claims that:
>
> > io_method=io_uring is available on Linux 5.1+.
>
> Attempting to set io_method to io_uring on versions without io_uring at all
> (below 5.1) causes the server to graciously report that "[the k]ernel does
> not support io_uring".
> But setting io_uring on kernel versions between 5.1 and 5.6 sees the server
> start but connections failing with EINVAL.
>
> ```
> [ec2-user(at)ip-172-31-56-49 postgres]$ psql postgres
> psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed:
> FATAL: could not read blocks 0..0 in file "global/1260": Invalid argument
> ```
>
> It seems like the implementation in postgres leverages non-vectored I/O
> (i.e. io_uring_prep_read and io_uring_prep_write) in
> src/backend/storage/aio/method_io_uring.c.
>
> But, these opcodes aren't actually available until kernel 5.6
> (https://github.com/axboe/liburing/issues/44)
> (https://man7.org/linux/man-pages/man2/io_uring_enter.2.html - under
> IORING_OP_READ, IORING_OP_WRITE).
>
> It seems kernel 5.4 LTS ended last month, but this seems worth clarifying,
> I
> think.
>
> --
>
> Precise reproduction example:
>
> OS details
> ```
> [ec2-user(at)ip-172-31-56-49 postgres]$ uname -a
> Linux ip-172-31-56-49.us-west-2.compute.internal
> 5.4.301-221.450.amzn2.x86_64 #1 SMP Tue Nov 18 16:40:04 UTC 2025 x86_64
> x86_64 x86_64 GNU/Linux
> ```
>
> Build postgres with liburing 2.8, set postgresql.conf with
> io_method=io_uring, log_min_messages = debug3.
>
> Postgres starts without complaint, but trying to connect will fail:
>
> ```
> [ec2-user(at)ip-172-31-56-49 postgres]$ psql postgres
> psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed:
> FATAL: could not read blocks 0..0 in file "global/1260": Invalid argument
> ```
> Inside logfile:
> ```
> 2026-01-02 22:35:43.862 UTC [8517] DEBUG: io 6400 |op invalid|target
> invalid|state HANDED_OUT : adding cb #1, id
> 2/aio_shared_buffer_readv_cb
> 2026-01-02 22:35:43.862 UTC [8517] DEBUG: io 6400 |op invalid|target
> smgr|state HANDED_OUT : adding cb #2, id 1/aio_md_readv_cb
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state DEFINED : calling cb #1
> 2/aio_shared_buffer_readv_cb->stage(0)
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state STAGED : staged (synchronous: 0, in_batch: 1)
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state SUBMITTED : wait_one io_gen: 3, ref_gen: 3, cycle 0
> 2026-01-02 22:35:43.863 UTC [8517] LOG: could not read blocks 0..0 in file
> "global/1262": Invalid argument
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state COMPLETED_IO : after shared completion: distilled result:
> (status ERROR, id 1, error_data: 22, result 0), raw_result: -22
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state COMPLETED_SHARED: after local completion: result: (status ERROR,
> id 1, error_data 22, result 0), raw_result: -22
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: drained 1/1, now expecting 0
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: io 6400 |op invalid|target
> invalid|state IDLE : wait_one io_gen: 4, ref_gen: 3, cycle 0
> 2026-01-02 22:35:43.863 UTC [8517] DEBUG: wait_one with 0 sleeps
> 2026-01-02 22:35:43.863 UTC [8517] ERROR: could not read blocks 0..0 in
> file "global/1262": Invalid argument
> 2026-01-02 22:35:44.057 UTC [8512] DEBUG: assigned pm child slot 1 for
> backend
> 2026-01-02 22:35:44.058 UTC [8512] DEBUG: forked new client backend,
> pid=8521 socket=151
> 2026-01-02 22:35:44.058 UTC [8521] DEBUG: InitPostgres
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op invalid|target
> invalid|state HANDED_OUT : adding cb #1, id
> 2/aio_shared_buffer_readv_cb
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op invalid|target
> smgr|state HANDED_OUT : adding cb #2, id 1/aio_md_readv_cb
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op readv|target
> smgr|state DEFINED : calling cb #1
> 2/aio_shared_buffer_readv_cb->stage(0)
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op readv|target
> smgr|state STAGED : staged (synchronous: 0, in_batch: 1)
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op readv|target
> smgr|state SUBMITTED : wait_one io_gen: 1, ref_gen: 1, cycle 0
> 2026-01-02 22:35:44.059 UTC [8521] LOG: could not read blocks 0..0 in file
> "global/1260": Invalid argument
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op readv|target
> smgr|state COMPLETED_IO : after shared completion: distilled result:
> (status ERROR, id 1, error_data: 22, result 0), raw_result: -22
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op readv|target
> smgr|state COMPLETED_SHARED: after local completion: result: (status ERROR,
> id 1, error_data 22, result 0), raw_result: -22
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: drained 1/1, now expecting 0
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: io 0 |op invalid|target
> invalid|state IDLE : wait_one io_gen: 2, ref_gen: 1, cycle 0
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: wait_one with 0 sleeps
> 2026-01-02 22:35:44.059 UTC [8521] FATAL: could not read blocks 0..0 in
> file "global/1260": Invalid argument
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: shmem_exit(1): 5
> before_shmem_exit callbacks to make
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: shmem_exit(1): 7 on_shmem_exit
> callbacks to make
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: proc_exit(1): 2 callbacks to
> make
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: exit(1)
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: shmem_exit(-1): 0
> before_shmem_exit callbacks to make
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: shmem_exit(-1): 0 on_shmem_exit
> callbacks to make
> 2026-01-02 22:35:44.059 UTC [8521] DEBUG: proc_exit(-1): 0 callbacks to
> make
> 2026-01-02 22:35:44.060 UTC [8512] DEBUG: releasing pm child slot 1
> 2026-01-02 22:35:44.060 UTC [8512] DEBUG: client backend (PID 8521) exited
> with exit code 1
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op invalid|target
> invalid|state HANDED_OUT : adding cb #1, id
> 2/aio_shared_buffer_readv_cb
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op invalid|target
> smgr|state HANDED_OUT : adding cb #2, id 1/aio_md_readv_cb
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state DEFINED : calling cb #1
> 2/aio_shared_buffer_readv_cb->stage(0)
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state STAGED : staged (synchronous: 0, in_batch: 1)
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state SUBMITTED : wait_one io_gen: 4, ref_gen: 4, cycle 0
> 2026-01-02 22:35:44.863 UTC [8517] LOG: could not read blocks 0..0 in file
> "global/1262": Invalid argument
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state COMPLETED_IO : after shared completion: distilled result:
> (status ERROR, id 1, error_data: 22, result 0), raw_result: -22
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op readv|target
> smgr|state COMPLETED_SHARED: after local completion: result: (status ERROR,
> id 1, error_data 22, result 0), raw_result: -22
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: drained 1/1, now expecting 0
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: io 6400 |op invalid|target
> invalid|state IDLE : wait_one io_gen: 5, ref_gen: 4, cycle 0
> 2026-01-02 22:35:44.863 UTC [8517] DEBUG: wait_one with 0 sleeps
> 2026-01-02 22:35:44.863 UTC [8517] ERROR: could not read blocks 0..0 in
> file "global/1262": Invalid argument
> ```
>
>