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Re: Synchronous replication, reading WAL for sending

From: "Pavan Deolasee" <pavan(dot)deolasee(at)gmail(dot)com>
To: "Fujii Masao" <masao(dot)fujii(at)gmail(dot)com>
Cc: "Heikki Linnakangas" <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, "Simon Riggs" <simon(at)2ndquadrant(dot)com>, "Pavan Deolasee" <pavan(dot)deolasee(at)enterprisedb(dot)com>
Subject: Re: Synchronous replication, reading WAL for sending
Date: 2008-12-24 10:10:34
Message-ID: 2e78013d0812240210m567e4948jb6c15896b93d725@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Dec 24, 2008 at 3:01 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
>
> Good question. If streaming and copying are performed parallelly,
> such situation doesn't happen because the speed to generate xlog
> also depends on streaming. This is a price to pay. I think that the
> serial operations would need a "pace maker". And, I don't know
> better pace maker than concurrent streaming.
>

These operations need not be even parallel. My apologies if this has
been discussed before, but what we are talking about is just a stream
of WAL starting at some LSN. The only difference is that the LSN
itself may be in buffers or in the files. So walsender would send as
much as it can from the files and then switch to read from buffers.

Also, I think you are underestimating the power of network for most
practical purposes. Networks are usually not bottlenecks unless we are
talking about slow WAN setups which I am not sure how common for PG
users.
.
>
> Right. Completely high load setup is probably impossible.

If that's the case, I don't think you need to worry too much about
network or the walsender being a bottleneck for initial sync up (and
note that we are only talking about WAL sync up and not the base
backup).

Thanks,
Pavan

-- 
Pavan Deolasee
EnterpriseDB     http://www.enterprisedb.com

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