Re: AIO v2.5

Hi,

Attached is v2.6 of the AIO patchset.

Relative to 2.5 I:

- Improved the split between subsystem initialization and main AIO commit, as
well as the one between worker infrastructure and io_method=worker

Seemed worth as the only one voicing an opinion about squashing those
commits was opposed.

- Added a lot more comments to aio.h/aio_internal.h. I think just about
anything that should conceivably have a comment has one.

- Reordered fields in PgAioHandle to waste less due to padding

- Narrowed a few *count fields, they were 64bit without ever being al to reach
that

- Used aio_types.h more widely, instead of "manual" forward declarations. This
required moving a few typedefs to aio_types.h

- Substantial commit message improvements.

- Removed the pg_aios.error_desc column, due to:
https://postgr.es/m/qzxq6mqqozctlfcg2kg5744gmyubicvuehnp4a7up472thlvz2%40y5xqgd5wcwhw

- Reordered the commits slightly, to put the README just after the
smgr.c/md.c/... support, as the readme references those in the examples

- Stopped creating backend-local io_uring instances, that is vestigial for
now. We likely will want to reintroduce them at some point (e.g. for network
IO), but we can do that at that time.

- There were a lot of duplicated codepaths in bufmgr.c support for AIO due to
temp tables. I added a few commits refactoring the temp buffers state
management to look a lot more like the shared buffer code.

I'm not sure that that's the best path, but they all seemed substantial
improvements on their own.

- putting io_method in PG_TEST_INITDB_EXTRA_OPTS previously broke a test,
because Cluster::init() puts PG_TEST_INITDB_EXTRA_OPTS after the # options
specified by ->extra. I now worked around that by appending the io method to
a local PG_TEST_INITDB_EXTRA_OPTS, but brrr.

- The tracepoint for read completion omitted the fact that it was a temp
table, if so.

- Fixed some duplicated function decls, due to a misresolved merge-conflict

Current state:

- 0001, 0002 - core AIO - IMO pretty much ready

- 0003, 0004 - IO worker - same

- 0005, 0006 - io_uring support - close, but we need to do something about
set_max_fds(), which errors out spuriously in some cases

- 0007 - smgr/md/fd.c readv support - seems quite close, but might benefit from
another pass through

- 0008 - README - I think it's good, but I'm probably not seeing the trees for
the forest anymore

- 0009 - pg_aios view - naming not resolved, docs missing

- 0010 to 0014 - from another thread, just included here due to a dependency

- 0016 to 0020 - cleanups for temp buffers code - I just wrote these to clean
up the code before making larger changes, needs review

- 0021 - keep BufferDesc refcount up2date for temp buffers - I think that's
pretty much ready, but depends on earlier patches

- 0022 - bufmgr readv AIO suppot - some naming, some code duplication needs to
be resolved, but otherwise quite close

- 0023 - use AIO in StartReadBuffers() - perhaps a bit of polishing needed

- 0024 - adjust read_stream.c for AIO - I think Thomas has a better patch for
this in the works

- 0025 - tests for AIO - I think it's reasonable, unless somebody objects to
exporting a few bufmgr.c functions to the test

- the rest: Not for 18

Greetings,

Andres Freund