Horton

Re: Use pg_pwritev_with_retry() instead of write() in dir_open_for_write() to avoid partial writes?

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Nathan Bossart <nathandbossart@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2023-02-12T17:31:36Z
Lists: pgsql-hackers 
Hi,

On 2023-02-12 19:59:00 +0530, Bharath Rupireddy wrote:
> Thanks for looking at it. We know that we don't change the zbuffer in
> the function, so can we avoid static const and have just a static
> variable, like the attached
> v1-0001-Use-static-variable-to-avoid-memset-calls-in-pg_p.patch? Do
> you see any problem with it?

Making it static const is better, because it allows the memory for the
variable to be put in a readonly section.


>  	/* Prepare to write out a lot of copies of our zero buffer at once. */
>  	for (i = 0; i < lengthof(iov); ++i)
>  	{
> -		iov[i].iov_base = zbuffer.data;
> +		iov[i].iov_base = (void *) (unconstify(PGAlignedBlock *, &zbuffer)->data);
>  		iov[i].iov_len = zbuffer_sz;
>  	}

Another thing: I think we should either avoid iterating over all the IOVs if
we don't need them, or, even better, initialize the array as a constant, once.

Greetings,

Andres Freund

Commits

  1. Revise pg_pwrite_zeros()

  2. Use pg_pwrite_zeros() in walmethods.c

  3. Introduce pg_pwrite_zeros() in fileutils.c

  4. Move pg_pwritev_with_retry() to src/common/file_utils.c

  5. Restore pg_pread and friends.

  6. Remove dead pread and pwrite replacement code.