Re: Question on pg_stat_io showing zero reads/writes for I/O workers

Hi,

On Fri, Oct 31, 2025 at 4:17 PM Shardul Borhade <shardul@dbtune.com> wrote:
>
> Hi team,
>
> I’m running into an issue with pg_stat_io. When I run the following query:
>
> SELECT backend_type, reads, writes, read_time
> FROM pg_stat_io
> WHERE backend_type LIKE '%io%';
>
> I consistently get:
>
>  backend_type | reads | writes | read_time
> --------------+-------+--------+-----------
>  io worker    |     0 |      0 |         0
>  io worker    |     0 |      0 |         0
>  io worker    |     0 |      0 |         0
>  io worker    |     0 |      0 |         0
>  io worker    |     0 |      0 |         0
>  io worker    |     0 |      0 |         0
>  io worker    |       |      0 |
>  io worker    |     0 |      0 |         0
> (8 rows)
>
> I tried running a heavy sequential scan on a 55 GB table as well as a bitmap heap scan, but the reads and writes columns still show zero.
>
> However, I can clearly observe performance differences when I tune the io_workers configuration, so I believe they are active.
>
> Am I missing something here? Could someone please help me understand why the stats aren’t being reflected in pg_stat_io? Do I need to enable any other server parameter to log this information?

When your client backend issues a query:

1. Your backend calls `AsyncReadBuffers()` and counts the IO stats
2. The IO worker performs the actual I/O but doesn't increment its own counters
3. Stats accumulate under "client backend", not "io worker"

Check your *client backend* stats instead:

```sql
You should see reads/writes here:

SELECT backend_type, context, object, reads, writes
FROM pg_stat_io
WHERE backend_type = 'client backend'
  AND reads > 0
ORDER BY reads DESC;

-- Verify IO workers exist:

SELECT pid, backend_type, wait_event
FROM pg_stat_activity
WHERE backend_type = 'io worker';


Quick Test

-- Reset stats

SELECT pg_stat_reset_shared('io');

-- Run your heavy scan

SELECT COUNT(*) FROM your_large_table;

-- Check client backend stats (should increase)

SELECT SUM(reads) as total_reads
FROM pg_stat_io
WHERE backend_type = 'client backend';

-- Check IO worker stats (will remain zero)

SELECT SUM(reads) as total_reads
FROM pg_stat_io
WHERE backend_type = 'io worker';

See source code:

- `src/backend/storage/buffer/bufmgr.c` (AsyncReadBuffers, line 1938)
- stats counted at submission
- `src/backend/utils/activity/pgstat_io.c` (pgstat_count_io_op, line
74) - uses submitter's MyBackendType
- `src/backend/storage/aio/aio_io.c` (pgaio_io_perform_synchronously)
- IO worker doesn't call stat functions

However, this behavior seems not great here. If the above analysis is
sound, should we add something like this to not track i/o worker
// In pgstat_tracks_io_bktype()
case B_IO_WORKER:
    return false;  // Don't show zero-value rows
or track actual IO worker stats?

Best,
Xuneng