Date: Tue, 21 Jan 2025 10:13:14 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Andy Fan <zhihuifan1213@163.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, Maxim Orlov <orlovmg@gmail.com>,
	Pavel Borisov <pashkin.elfe@gmail.com>,
	"Bossart, Nathan" <bossartn@amazon.com>,
	Maxim Orlov <m.orlov@postgrespro.ru>,
	Andres Freund <andres@anarazel.de>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: Pre-allocating WAL files
Message-ID: <Z4_Hmj2R8NXnAI-K@nathan>
References: 
 <163352276350.15435.3988688227048304232.pgcf@coridan.postgresql.org>
 <CB15BEBD-98FC-4E72-86AE-513D34014176@amazon.com>
 <CALT9ZEG+R9ne6xTyEVSiLhZ7wJAQ1Jq-9XVZs=zbd_jQJyPTdA@mail.gmail.com>
 <CACG=ezb5Os_ohi1caZJLKsGJc-nSSqGXFrMjmKw4qN8fG_=NEg@mail.gmail.com>
 <20220301144044.GG25269@telsasoft.com>
 <20220301150704.GA1030078@nathanxps13>
 <20220317231212.GB1415284@nathanxps13>
 <20220408203003.GA1630183@nathanxps13>
 <87sepc4zcg.fsf@163.com>
 <Z4_C04ZJ_SymvYes@nathan>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Z4_C04ZJ_SymvYes@nathan>
Archived-At: <https://www.postgresql.org/message-id/Z4_Hmj2R8NXnAI-K%40nathan>
Precedence: bulk

On Tue, Jan 21, 2025 at 09:52:51AM -0600, Nathan Bossart wrote:
> On Tue, Jan 21, 2025 at 03:31:27AM +0000, Andy Fan wrote:
>> 3. Why is the purpose of preallocated_segments directory? what in my
>> mind is we just prellocate the normal filename so that XLogWrite could
>> open it directly. This is same as what wal_recycle does and we can reuse
>> the same strategy to clean up them if they are not needed anymore.
> 
> The purpose is to limit the use of pre-allocated segments to only
> situations where WAL recycling is not sufficient.  Basically, if writing a
> record would require a new segment to be created, we can quickly pull a
> pre-allocated one instead of creating it ourselves.  Besides simplifying
> matters, this prevents a lot of unnecessary pre-allocation, since many
> workloads will almost never need anything beyond the recycled segments.

That being said, it would be nice to avoid the fsync() overhead to move a
pre-allocated WAL into place.  My first instinct is that would be
substantially more complicated and may not actually improve matters all
that much, but I agree that it's worth exploring.

-- 
nathan