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From: Guillaume Lelarge <guillaume@lelarge.info>
Date: Mon, 20 Jan 2025 11:31:44 +0100
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Subject: Re: wal_compression
To: Fabrice Chapuis <fabrice636861@gmail.com>
Cc: pgsql-admin@lists.postgresql.org
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Hi,

Le lun. 20 janv. 2025 =C3=A0 11:23, Fabrice Chapuis <fabrice636861@gmail.co=
m> a
=C3=A9crit :

> In Postgres 17 doc, for wal_compression parameter it's written:
> "Turning this parameter on can reduce the WAL volume without increasing
> the risk of unrecoverable data corruption"
> If wal_compression =3D on then when a full page is written in the wal, it
> will be compressed. What I do not understand is that the wal file keeps t=
he
> same size 16Mb. What is the interest to activate wal_compression?
>
>
A WAL file contains some records. Usually a lot of records. If a record is
compressed, it will take less space in the WAL file. Hence, this WAL file
will contain more records, and you will need less WAL files for the same
activity.


--=20
Guillaume.

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<div dir=3D"ltr"><div dir=3D"ltr">Hi,<br></div><br><div class=3D"gmail_quot=
e gmail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">Le=C2=A0lun.=
 20 janv. 2025 =C3=A0=C2=A011:23, Fabrice Chapuis &lt;<a href=3D"mailto:fab=
rice636861@gmail.com">fabrice636861@gmail.com</a>&gt; a =C3=A9crit=C2=A0:<b=
r></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex=
;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr">=
<div dir=3D"ltr"><div><span style=3D"color:rgb(0,0,0);font-family:&quot;Ope=
n Sans&quot;,sans-serif;font-size:14.4px">In Postgres 17 doc, for wal_compr=
ession parameter it&#39;s written:</span></div><div dir=3D"ltr"><span style=
=3D"color:rgb(0,0,0);font-family:&quot;Open Sans&quot;,sans-serif;font-size=
:14.4px">&quot;Turning this parameter on can reduce the WAL volume without =
increasing the risk of unrecoverable data corruption&quot;<br>If=C2=A0</spa=
n><span style=3D"color:rgb(0,0,0);font-family:&quot;Open Sans&quot;,sans-se=
rif;font-size:14.4px">wal_compression</span><span style=3D"color:rgb(0,0,0)=
;font-family:&quot;Open Sans&quot;,sans-serif;font-size:14.4px">=C2=A0=3D o=
n then when a full page is written in the wal, it will be compressed. What =
I do not understand is that the wal file keeps the same size 16Mb. What is =
the interest to activate wal_compression?</span></div><div dir=3D"ltr"><spa=
n style=3D"color:rgb(0,0,0);font-family:&quot;Open Sans&quot;,sans-serif;fo=
nt-size:14.4px"><br></span></div></div></div></blockquote><div><br></div><d=
iv>A WAL file contains some records. Usually a lot of records. If a record =
is compressed, it will take less space in the WAL file. Hence, this WAL fil=
e will contain more records, and you will need less WAL files for the same =
activity.</div></div><div><br clear=3D"all"></div><br><span class=3D"gmail_=
signature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signature">=
<div dir=3D"ltr"><div><div dir=3D"ltr"><div>Guillaume.<br></div></div></div=
></div></div></div>

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