Message-ID: <e40e61e6671d02ce79512439054bda9932bef8c1.camel@cybertec.at>
Subject: Re: How to prioritise walsender reading from pg_wal over WAL writes?
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Alexey Bashtanov <bashtanov@imap.cc>
Cc: pgsql-performance@postgresql.org
Date: Wed, 18 Nov 2020 13:11:23 +0100
In-Reply-To: <774ec1b8-c7c6-cfa2-098d-5c6a32dcfc1a@imap.cc>
References: <774ec1b8-c7c6-cfa2-098d-5c6a32dcfc1a@imap.cc>
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On Wed, 2020-11-18 at 11:54 +0000, Alexey Bashtanov wrote:
> If it helps, here's the details of the hardware config.
> The controller is AVAGO MegaRAID SAS 9361-4i,
> the SSDs are INTEL SSDSC2KG960G8 (configured as a raid1).
> Current scheduler used is deadline.
> Currently XFS is mounted without nobarriers, but I'm going to set that when there's high load next time to see how it affects throughput.
> 
> Properties of the array
> VD6 Properties :
> ==============
> Strip Size = 256 KB
> Number of Blocks = 1874329600
> VD has Emulated PD = Yes
> Span Depth = 1
> Number of Drives Per Span = 2
> Write Cache(initial setting) = WriteBack
> Disk Cache Policy = Disk's Default
> Encryption = None
> Data Protection = Disabled
> Active Operations = None
> Exposed to OS = Yes
> Creation Date = 28-10-2020
> Creation Time = 05:17:16 PM
> Emulation type = default
> Cachebypass size = Cachebypass-64k
> Cachebypass Mode = Cachebypass Intelligent
> Is LD Ready for OS Requests = Yes
> SCSI NAA Id = 600605b00af03650272c641ca30c3196

Why??

WAL buffers has the most recent information, so that would result in unnecessary delay and I/O.

You'd have to hack the code, but I wonder what leads you to this interesting requirement.

Yours,
Laurenz Albe
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