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Introduce io_max_combine_limit.

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The existing io_combine_limit can be changed by users.  The new
io_max_combine_limit is fixed at server startup time, and functions as a
silent clamp on the user setting.  That in itself is probably quite
useful, but the primary motivation is:

aio_init.c allocates shared memory for all asynchronous IOs including
some per-block data, and we didn't want to waste memory you'd never used
by assuming they could be up to PG_IOV_MAX.  This commit already halves
the size of 'AioHandleIov' and 'AioHandleData'.  A follow-up commit can
now expand PG_IOV_MAX without affecting that.

Since our GUC system doesn't support dependencies or cross-checks
between GUCs, the user-settable one now assigns a "raw" value to
io_combine_limit_guc, and the lower of io_combine_limit_guc and
io_max_combine_limit is maintained in io_combine_limit.

Reviewed-by: Andres Freund <andres@anarazel.de> (earlier version)
Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com
This commit is contained in:
Thomas Munro
2025-03-19 11:40:56 +13:00
parent 17d8bba6da
commit 10f6646847
8 changed files with 72 additions and 15 deletions
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2
src/backend/utils/misc/postgresql.conf.sample
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@ -200,6 +200,8 @@
#backend_flush_after = 0 # measured in pages, 0 disables
#effective_io_concurrency = 16 # 1-1000; 0 disables prefetching
#maintenance_io_concurrency = 16 # 1-1000; 0 disables prefetching
#io_max_combine_limit = 128kB # usually 1-32 blocks (depends on OS)
# (change requires restart)
#io_combine_limit = 128kB # usually 1-32 blocks (depends on OS)
#io_method = worker # worker, sync (change requires restart)