Horton

Re: AIO v2.5

Jakub Wartak <jakub.wartak@enterprisedb.com>

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From: Jakub Wartak <jakub.wartak@enterprisedb.com>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>
Date: 2025-03-06T11:36:43Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. aio: Fix assertion, clarify README

  2. aio: Fix reference to outdated name

  3. aio: Fix possible state confusions due to interrupt processing

  4. aio: Improve debug logging around waiting for IOs

  5. aio: Fix crash potential for pg_aios views due to late state update

  6. Increase BAS_BULKREAD based on effective_io_concurrency

  7. localbuf: Add Valgrind buffer access instrumentation

  8. aio: Make AIO more compatible with valgrind

  9. aio: Avoid spurious coverity warning

  10. tests: Fix incompatibility of test_aio with *_FORCE_RELEASE

  11. tests: Cope with WARNINGs during failed CREATE DB on windows

  12. aio: Add errcontext for processing I/Os for another backend

  13. aio: Add README.md explaining higher level design

  14. aio: Minor comment improvements

  15. aio: Add test_aio module

  16. aio: Add pg_aios view

  17. docs: Add acronym and glossary entries for I/O and AIO

  18. Enable IO concurrency on all systems

  19. read_stream: Introduce and use optional batchmode support

  20. docs: Reframe track_io_timing related docs as wait time

  21. bufmgr: Use AIO in StartReadBuffers()

  22. bufmgr: Implement AIO read support

  23. aio: Add WARNING result status

  24. Let caller of PageIsVerified() control ignore_checksum_failure

  25. pgstat: Allow checksum errors to be reported in critical sections

  26. Add errhint_internal()

  27. localbuf: Track pincount in BufferDesc as well

  28. aio, bufmgr: Comment fixes/improvements

  29. Fix mis-attribution of checksum failure stats to the wrong database

  30. aio: Implement support for reads in smgr/md/fd

  31. aio: Add io_method=io_uring

  32. aio: Add liburing dependency

  33. aio: Rename pgaio_io_prep_* to pgaio_io_start_*

  34. aio: Pass result of local callbacks to ->report_return

  35. aio: Be more paranoid about interrupts

  36. Redefine max_files_per_process to control additionally opened files

  37. aio: Change prefix of PgAioResultStatus values to PGAIO_RS_

  38. bufmgr: Improve stats when a buffer is read in concurrently

  39. aio: Add io_method=worker

  40. aio: Infrastructure for io_method=worker

  41. aio: Add core asynchronous I/O infrastructure

  42. aio: Basic subsystem initialization

  43. tests: Expand temp table tests to some pin related matters

  44. localbuf: Introduce FlushLocalBuffer()

  45. localbuf: Introduce TerminateLocalBufferIO()

  46. localbuf: Fix dangerous coding pattern in GetLocalVictimBuffer()

  47. localbuf: Introduce StartLocalBufferIO()

  48. localbuf: Introduce InvalidateLocalBuffer()

  49. Allow lwlocks to be disowned

  50. Make jsonb casts to scalar types translate JSON null to SQL NULL.

  51. bufmgr/smgr: Don't cross segment boundaries in StartReadBuffers()

  52. Use aux process resource owner in walsender

  53. bufmgr: Return early in ScheduleBufferTagForWriteback() if fsync=off

On Tue, Mar 4, 2025 at 8:00 PM Andres Freund <andres@anarazel.de> wrote:

> Attached is v2.5 of the AIO patchset.
[..]
Hi, Thanks for working on this!

> Questions:
>
> - My current thinking is that we'd set io_method = worker initially - so we
>   actually get some coverage - and then decide whether to switch to
>   io_method=sync by default for 18 sometime around beta1/2. Does that sound
>   reasonable?

IMHO, yes, good idea. Anyway final outcomes partially will depend on
how many other stream-consumers be committed, right?

> - Three of the commits in the series really are just precursor commits to
>   their subsequent commits, which I found helpful for development and review,
>   namely:
>
>   - aio: Basic subsystem initialization
>   - aio: Skeleton IO worker infrastructure
>   - aio: Add liburing dependency
>
>   Not sure if it's worth keeping these separate or whether they should just be
>   merged with their "real commit".

For me it was easier to read those when they are separate.

> - Right now this series defines PGAIO_VERBOSE to 1. That's good for debugging,
>   but all the ereport()s add a noticeable amount of overhead at high IO
>   throughput (at multiple gigabytes/second), so that's probably not right
>   forever.  I'd leave this on initially and then change it to default to off
>   later.  I think that's ok?

+1, hopefully nothing is recording/logging/running with
log_min_messages>=debug3 because only then it starts to be visible.

> - To allow io_workers to be PGC_SIGHUP, and to eventually allow to
>   automatically in/decrease active workers, the max number of workers (32) is
>   always allocated. That means we use more semaphores than before. I think
>   that's ok, it's not 1995 anymore.  Alternatively we can add a
>   "io_workers_max" GUC and probe for it in initdb.

Wouldn't that matter only on *BSDs?

BTW I somehow cannot imagine someone saturating >= 32 workers (if one
does, better to switch to uring anyway?), but I have a related
question about closing fd by those workers.

> - pg_stat_aios currently has the IO Handle flags as dedicated columns. Not
>   sure that's great?
>
>   They could be an enum array or such too? That'd perhaps be a bit more
>   extensible? OTOH, we don't currently use enums in the catalogs and arrays
>   are somewhat annoying to conjure up from C.

s/pg_stat_aios/pg_aios/ ? :^) It looks good to me as it is. Anyway it
is a debugging view - perhaps mark it as such in the docs - so there
is no stable API for that and shouldn't be queried by any software
anyway.

> - Documentation for pg_stat_aios.

pg_aios! :)

So, I've taken aio-2 branch from Your's github repo for a small ride
on legacy RHEL 8.7 with dm-flakey to inject I/O errors. This is more a
question: perhaps IO workers should auto-close fd on errors or should
we use SIGUSR2 for it? The scenario is like this:

#dm-dust is not that available even on modern distros(not always
compiled), but flakey seemed to work on 4.18.x:
losetup /dev/loop0 /dd.img
mkfs.ext4 -j /dev/loop0
mkdir /flakey
mount /dev/loop0 /flakey # for now it will work
mkdir /flakey/tblspace
chown postgres /flakey/tblspace
chmod 0700 /flakey/tblspace
CREATE TABLESPACE test1 LOCATION '/flakey/tblspace'
CREATE TABLE on t1fail on that test1 tablespace + INSERT SOME DATA
pg_ctl stop
umount /flakey
echo "0 `blockdev --getsz /dev/loop0` flakey /dev/loop0 0 1 1" |
dmsetup create flakey # after 1s start throwing IO errors
mount /dev/mapper/flakey /flakey
#might even say: mount: /flakey: can't read superblock on /dev/mapper/flakey.
mount /dev/mapper/flakey /flakey
pg_ctl start

and then this will happen:

postgres=# insert into t1fail select generate_series(1000001, 2000001);
ERROR:  could not read blocks 0..1 in file
"pg_tblspc/24579/PG_18_202503031/5/24586_fsm": Input/output error
postgres=# insert into t1fail select generate_series(1000001, 2000001);
ERROR:  could not read blocks 0..1 in file
"pg_tblspc/24579/PG_18_202503031/5/24586_fsm": Input/output error
postgres=# insert into t1fail select generate_series(1000001, 2000001);
ERROR:  could not read blocks 0..1 in file
"pg_tblspc/24579/PG_18_202503031/5/24586_fsm": Input/output error

postgres=# insert into t1fail select generate_series(1000001, 2000001);
ERROR:  could not open file
"pg_tblspc/24579/PG_18_202503031/5/24586_vm": Read-only file system

so usual stuff with kernel remounting it RO, but here's the dragon
with io_method=worker:

# mount -o remount,rw /flakey/
mount: /flakey: cannot remount /dev/mapper/flakey read-write, is
write-protected.
# umount /flakey # to fsck or just mount rw again
umount: /flakey: target is busy.
# lsof /flakey/
COMMAND     PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
postgres 103483 postgres   14u   REG  253,2 36249600   17
/flakey/tblspace/PG_18_202503031/5/24586
postgres 103484 postgres    6u   REG  253,2 36249600   17
/flakey/tblspace/PG_18_202503031/5/24586
postgres 103485 postgres    6u   REG  253,2 36249600   17
/flakey/tblspace/PG_18_202503031/5/24586

Those 10348[345] are IO workers, they have still open fds and there's
no way to close those without restart -- well without close()
injection probably via gdb.   pg_terminate_backend() on those won't
work. The only thing that works seems to be sending SIGUSR2, but is
that safe [there could be some errors after pwrite() ] ? With
io_worker=sync just quitting the backend of course works. Not sure
what your thoughts are because any other bgworker could be having open
fds there. It's a very minor thing. Otherwise that outage of separate
tablespace (rarely used) would potentially cause inability to fsck
there and lower the availability of the DB (due to potential restart
required). I'm thinking especially of scenarios where lots of schemas
are used with lots of tablespaces OR where temp_tablespace is employed
for some dedicated (fast/furious/faulty) device. So I'm hoping SIGUSR2
is enough right (4231f4059e5e54d78c56b904f30a5873da88e163 seems to be
doing it anyway) ?

BTW: While at this, I've tried amcheck/pg_surgery for 1 min and they
both seem to work.

-J.