Horton

Re: pg_verify_checksums and -fno-strict-aliasing

Fabien COELHO <coelho@cri.ensmp.fr>

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From: Fabien COELHO <coelho@cri.ensmp.fr>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Michael Paquier <michael@paquier.xyz>, Michael Banck <michael.banck@credativ.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-09-01T05:24:38Z
Lists: pgsql-hackers 
Hello,

>> Okay, for the memo replay_image_masked and master_image_masked
>> in xlog.c could make use of the new structure.  SetWALSegSize in
>> pg_standby.c and WriteEmptyXLOG in pg_resetwal.c, and pg_upgrade's
>> file.c could also be patched.
>
> I intentionally didn't change replay_image_masked/master_image_masked
> to use statically-allocated buffers.  Since, AFAICS, those aren't
> needed in most backend processes, they'd just be eating 16KB of
> per-process data space to no purpose.
>
> The others you mention could be changed, probably, but I didn't
> bother as they didn't seem performance-critical.

I'd go for having just one same approach everywhere, for code base 
homogeneity.

>>> +typedef union PGAlignedBuffer
>
>> One complain I have is about the name of those structures.  Perhaps
>> PGAlignedBlock and PGAlignedXlogBlock make more sense?
>
> Don't have a strong preference, anybody else have an opinion?

I like "Block" better, because it's more precise.

> (I also wondered whether to use "WAL" instead of "XLog" in that
> struct name, but it seems like we've mostly stuck with "xlog"
> in internal C names.)

Best to blend with the surrounding code in the header file?

-- 
Fabien.

Commits

  1. Avoid using potentially-under-aligned page buffers.

  2. Code review for pg_verify_checksums.c.

  3. Make checksum_impl.h safe to compile with -fstrict-aliasing.