Horton

Re: Direct I/O

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Noah Misch <noah@leadboat.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, pgsql-hackers@postgresql.org
Date: 2023-04-08T18:08:16Z
Lists: pgsql-hackers

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Hi,

On 2023-04-07 23:04:08 -0700, Andres Freund wrote:
> There were some failures in CI (e.g. [1] (and perhaps also bf, didn't yet
> check), about "no unpinned buffers available".  I was worried for a moment
> that this could actually be relation to the bulk extension patch.
> 
> But it looks like it's older - and not caused by direct_io support (except by
> way of the test existing). I reproduced the issue locally by setting s_b even
> lower, to 16 and made the ERROR a PANIC.
>
> [backtrace]
> 
> If you look at log_newpage_range(), it's not surprising that we get this error
> - it pins up to 32 buffers at once.
> 
> Afaics log_newpage_range() originates in 9155580fd5fc, but this caller is from
> c6b92041d385.
> 
> 
> It doesn't really seem OK to me to unconditionally pin 32 buffers. For the
> relation extension patch I introduced LimitAdditionalPins() to deal with this
> concern. Perhaps it needs to be exposed and log_newpage_buffers() should use
> it?
> 
> 
> Do we care about fixing this in the backbranches? Probably not, given there
> haven't been user complaints?

Here's a quick prototype of this approach. If we expose LimitAdditionalPins(),
we'd probably want to add "Buffer" to the name, and pass it a relation, so
that it can hand off LimitAdditionalLocalPins() when appropriate? The callsite
in question doesn't need it, but ...

Without the limiting of pins the modified 004_io_direct.pl fails 100% of the
time for me.

Presumably the reason it fails occasionally with 256kB of shared buffers
(i.e. NBuffers=32) is that autovacuum or checkpointer briefly pins a single
buffer. As log_newpage_range() thinks it can just pin 32 buffers
unconditionally, it fails in that case.

Greetings,

Andres Freund

Commits

  1. Rename hook functions for debug_io_direct to match variable name.

  2. Rename io_direct to debug_io_direct.

  3. Skip the 004_io_direct.pl test if a pre-flight check fails.

  4. Use higher wal_level for 004_io_direct.pl.

  5. Skip \password TAP test on old IPC::Run versions

  6. Add io_direct setting (developer-only).

  7. Introduce PG_IO_ALIGN_SIZE and align all I/O buffers.

  8. Add palloc_aligned() to allow aligned memory allocations

  9. initdb: When running CREATE DATABASE, use STRATEGY = WAL_COPY.