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Subject: Re: Does my pg_xlog directory look right?
From: dstibrany@gmail.com
In-Reply-To: <54621.1463769300@sss.pgh.pa.us>
Date: Sun, 22 May 2016 17:40:18 -0400
Cc: pgsql-admin@postgresql.org
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Message-Id: <87F21501-4653-4659-9740-4ECB1AB0709E@gmail.com>
References: 
 <CAK17JmmMrHmWZ0Ypu52JwxrdQwPWfvwQn6DtufH4ju+bYderSw@mail.gmail.com>
 <54621.1463769300@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
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Sender: pgsql-admin-owner@postgresql.org

Thanks for the response and the pg_xlogdump tip.

Dave

> On May 20, 2016, at 2:35 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>=20
> Dave Stibrany <dstibrany@gmail.com> writes:
>> There are 98 WAL segments in the directory currently.
>=20
>> From the documentation, I understand that the number of WAL files can go=
 as
>> high as (3 * checkpoint_segments + 1) segment files, which would be 97 in
>> my case, but I seem to be consistently at this number or even higher
>> (sometimes over 110 files) and I don't think I've ever seen the directory
>> smaller than ~97 segment files.
>=20
> I don't believe there's any aggressive attempt to remove segment files
> after transient peak usage (what happens to them instead is explained
> below).  If you had many times the expected number of segments, I'd be
> worried, but these numbers sound pretty normal to me.
>=20
>> Regarding the ordering of segments, my understanding is that segment
>> 0000012000003720000008B would be written to after 0000001200000372000000=
8A,
>> but the timestamps (14:59 and 15:06, respectively) appear to show
>> otherwise. There are several cases like this. Does this mean that
>> 00000012000003720000008A has newer WAL records than
>> 00000012000003720000008B??
>=20
> No, it probably means that neither one has been written at all yet.
> Typically, when a WAL segment is deemed no longer needed, the file isn't
> physically removed but is merely renamed into place as a future segment.
> The idea is to reduce unnecessary filesystem work as we create and delete
> WAL segments.  You could check this out if you have pg_xlogdump at hand,
> by seeing whether the WAL file's first page header claims to belong to the
> segment indicated by the file name, or to some much-older segment.
>=20
>            regards, tom lane


--=20
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