Thread: pg_walfile_name uses XLByteToPrevSeg

pg_walfile_name uses XLByteToPrevSeg

From
Ashutosh Bapat
Date:
Hi All,
pg_walfile_name() returns the WAL file name corresponding to the given
WAL location. Per
https://www.postgresql.org/docs/14/functions-admin.html
---
pg_walfile_name ( lsn pg_lsn ) → text

Converts a write-ahead log location to the name of the WAL file
holding that location.
---

The function uses XLByteToPrevSeg() which gives the name of previous
WAL file if the location falls on the boundary of WAL segment. I find
it misleading since the given LSN will fall into the segment provided
by XLByteToSeg() and not XLBytePrevSeg().

And it gives some surprising results as well
---
#select pg_walfile_name('0/0'::pg_lsn);
     pg_walfile_name
--------------------------
 00000001FFFFFFFF000000FF
(1 row)
----

Comment in the code says
---
/*
 * Compute an xlog file name given a WAL location,
 * such as is returned by pg_stop_backup() or pg_switch_wal().
 */
Datum
pg_walfile_name(PG_FUNCTION_ARGS)
---
XLByteToPrevSeg() may be inline with the comment but I don't think
that's what is conveyed by the documentation at least.

--
Best Wishes,
Ashutosh



Re: pg_walfile_name uses XLByteToPrevSeg

From
Robert Haas
Date:
On Fri, Feb 4, 2022 at 9:05 AM Ashutosh Bapat
<ashutosh.bapat.oss@gmail.com> wrote:
> And it gives some surprising results as well
> ---
> #select pg_walfile_name('0/0'::pg_lsn);
>      pg_walfile_name
> --------------------------
>  00000001FFFFFFFF000000FF
> (1 row)
> ----

Yeah, that seems wrong.

-- 
Robert Haas
EDB: http://www.enterprisedb.com



Re: pg_walfile_name uses XLByteToPrevSeg

From
Nathan Bossart
Date:
On Fri, Feb 04, 2022 at 09:17:54AM -0500, Robert Haas wrote:
> On Fri, Feb 4, 2022 at 9:05 AM Ashutosh Bapat
> <ashutosh.bapat.oss@gmail.com> wrote:
>> And it gives some surprising results as well
>> ---
>> #select pg_walfile_name('0/0'::pg_lsn);
>>      pg_walfile_name
>> --------------------------
>>  00000001FFFFFFFF000000FF
>> (1 row)
>> ----
> 
> Yeah, that seems wrong.

It looks like it's been this way for a while (704ddaa).
pg_walfile_name_offset() has the following comment:

 * Note that a location exactly at a segment boundary is taken to be in
 * the previous segment.  This is usually the right thing, since the
 * expected usage is to determine which xlog file(s) are ready to archive.

I see a couple of discussions about this as well [0] [1].

[0] https://www.postgresql.org/message-id/flat/1154384790.3226.21.camel%40localhost.localdomain
[1] https://www.postgresql.org/message-id/flat/15952.1154827205%40sss.pgh.pa.us

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com



Re: pg_walfile_name uses XLByteToPrevSeg

From
Kyotaro Horiguchi
Date:
At Fri, 4 Feb 2022 14:50:57 -0800, Nathan Bossart <nathandbossart@gmail.com> wrote in 
> On Fri, Feb 04, 2022 at 09:17:54AM -0500, Robert Haas wrote:
> > On Fri, Feb 4, 2022 at 9:05 AM Ashutosh Bapat
> > <ashutosh.bapat.oss@gmail.com> wrote:
> >> And it gives some surprising results as well
> >> ---
> >> #select pg_walfile_name('0/0'::pg_lsn);
> >>      pg_walfile_name
> >> --------------------------
> >>  00000001FFFFFFFF000000FF
> >> (1 row)
> >> ----
> > 
> > Yeah, that seems wrong.
> 
> It looks like it's been this way for a while (704ddaa).
> pg_walfile_name_offset() has the following comment:
> 
>  * Note that a location exactly at a segment boundary is taken to be in
>  * the previous segment.  This is usually the right thing, since the
>  * expected usage is to determine which xlog file(s) are ready to archive.
> 
> I see a couple of discussions about this as well [0] [1].
> 
> [0] https://www.postgresql.org/message-id/flat/1154384790.3226.21.camel%40localhost.localdomain
> [1] https://www.postgresql.org/message-id/flat/15952.1154827205%40sss.pgh.pa.us

Yes, its the deliberate choice of design, or a kind of
questionable-but-unoverturnable decision.  I think there are many
external tools conscious of this behavior.

It is also described in the documentation.

https://www.postgresql.org/docs/current/functions-admin.html
> When the given write-ahead log location is exactly at a write-ahead
> log file boundary, both these functions return the name of the
> preceding write-ahead log file. This is usually the desired behavior
> for managing write-ahead log archiving behavior, since the preceding
> file is the last one that currently needs to be archived.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center



Re: pg_walfile_name uses XLByteToPrevSeg

From
Kyotaro Horiguchi
Date:
At Mon, 07 Feb 2022 13:21:53 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in 
> At Fri, 4 Feb 2022 14:50:57 -0800, Nathan Bossart <nathandbossart@gmail.com> wrote in 
> > On Fri, Feb 04, 2022 at 09:17:54AM -0500, Robert Haas wrote:
> > > On Fri, Feb 4, 2022 at 9:05 AM Ashutosh Bapat
> > > <ashutosh.bapat.oss@gmail.com> wrote:
> > >> And it gives some surprising results as well
> > >> ---
> > >> #select pg_walfile_name('0/0'::pg_lsn);
> > >>      pg_walfile_name
> > >> --------------------------
> > >>  00000001FFFFFFFF000000FF
> > >> (1 row)
> > >> ----
> > > 
> > > Yeah, that seems wrong.
> > 
> > It looks like it's been this way for a while (704ddaa).
> > pg_walfile_name_offset() has the following comment:
> > 
> >  * Note that a location exactly at a segment boundary is taken to be in
> >  * the previous segment.  This is usually the right thing, since the
> >  * expected usage is to determine which xlog file(s) are ready to archive.
> > 
> > I see a couple of discussions about this as well [0] [1].
> > 
> > [0] https://www.postgresql.org/message-id/flat/1154384790.3226.21.camel%40localhost.localdomain
> > [1] https://www.postgresql.org/message-id/flat/15952.1154827205%40sss.pgh.pa.us
> 
> Yes, its the deliberate choice of design, or a kind of
> questionable-but-unoverturnable decision.  I think there are many
> external tools conscious of this behavior.
> 
> It is also described in the documentation.
> 
> https://www.postgresql.org/docs/current/functions-admin.html
> > When the given write-ahead log location is exactly at a write-ahead
> > log file boundary, both these functions return the name of the
> > preceding write-ahead log file. This is usually the desired behavior
> > for managing write-ahead log archiving behavior, since the preceding
> > file is the last one that currently needs to be archived.

I forgot to mentino, but I don't think we need to handle the
wrap-around case of the function.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center