Sender: Gurjeet Singh <singh.gurjeet@gmail.com>
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Date: Sat, 29 Mar 2025 08:19:16 -0700
Message-Id: <D8SUNBF5CFW8.24B0ICY0SOFNI@singh.im>
Cc: "Euler Taveira" <euler@eulerto.com>, "Theodor Herlo"
 <t.herlo@proventa.de>, "Laurenz Albe" <laurenz.albe@cybertec.at>,
 "pgsql-docs@lists.postgresql.org" <pgsql-docs@lists.postgresql.org>
Subject: Re: Documentation for initdb option --waldir
From: "Gurjeet Singh" <gurjeet@singh.im>
To: "Robert Treat" <rob@xzilla.net>, "David G. Johnston"
 <david.g.johnston@gmail.com>
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On Sat Mar 29, 2025 at 7:26 AM PDT, Robert Treat wrote:
> On Thu, Mar 27, 2025 at 5:57=E2=80=AFPM David G. Johnston
> <david.g.johnston@gmail.com> wrote:
>> On Thu, Mar 27, 2025 at 2:28=E2=80=AFPM Robert Treat <rob@xzilla.net> wr=
ote:
>>> On Thu, Mar 27, 2025 at 12:06=E2=80=AFPM David G. Johnston
>>> <david.g.johnston@gmail.com> wrote:
>>> > I expanded upon the material regarding using different file systems a=
nd disks.
>>> >
>>> > I would like to add a similar "why" to the mount point recommendation=
 but don't know what that would be.  Suggestions welcomed.
>>> >
>>>
>>> I'm not sure I follow what you are asking for... but a non-performance
>>> reason to use a seperate mount point for pg_wal, even if the
>>> underlying storage is the same, would be for something like using
>>> filesystem snapshots to grab contents of the data directory without
>>> grabbing wal (which can be handled separately).
>>>
>>
>> If I mount the filesystem on disk2 to:  /mnt/disk2
>> Why do I need to create "/mnt/disk2/wal_files/" and point there instead =
of: "/mnt/disk2/"?
>>
>
> My immediate response to this was "because Postgres won't let you"
> which seemed unhelpful, and that I couldn't remember why was pretty
> unsatisfying, so I dug around in the source which was unhelpful but
> eventually came across this from
> https://www.postgresql.org/docs/devel/creating-cluster.html#CREATING-CLUS=
TER-MOUNT-POINTS
>
> "Best practice is to create a directory within the mount-point
> directory that is owned by the PostgreSQL user, and then create the
> data directory within that. This avoids permissions problems,..."
>
> Which I do remember having tried to do it directly and the OS
> complaining that my mount point wasn't owned by root and/or Postgres
> complaining that the xlog dir wasn't owned by Postgres, so I think
> this advice probably still holds.

I remembered an old advice related to lost+found being placed on mount
points, but couldn't remember the specific, so searched and found Tom's
advice for data directory. I feel that the advice would apply to WAL
directory, as well.