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Re: Warm standby server

From: Montaseri <montaseri(at)gmail(dot)com>
To: "Scott Whitney" <swhitney(at)journyx(dot)com>
Cc: "Simon Riggs" <simon(at)2ndquadrant(dot)com>, pgsql-admin(at)postgresql(dot)org
Subject: Re: Warm standby server
Date: 2008-06-26 18:20:57
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Lists: pgsql-admin
While I am not an expert on WAL, but again I question the merits of such
sophisticated HA configuration. Of course there are use cases for such
configs, but I am only advocating best price performance kind of mentality

As WAL writes the journals all the way down to the disk (ie write thru and
not write behind) before ack-ing toward the next step in a DB operation,
increasing the number of mirrors (one production, one on-site, one off-site,
I count 3 plexes here) will prolong each operation, with the following
exponentially increasing write latencies

production DB writes are at the rate of SCSI, SATA or system bus (30 MBps)
on-site DB writes are at the rate of LAN (10 MBps)
off-site DB writes are at the rate of WAN  (200 KBps)

Then if a three-way WAL writes is considered completed after the last WAN
write, then you have effectively lowered your performance to 200 KBytes per
sec writes. Now the gain. If the building gets destroyed, my data is
protected. Ok. what kind of business are we running in that building? ....
what is the rate of writes to database vs probability of building coming
down vs value of data from 2 hours ago vs 10 seconds ago.


On Thu, Jun 26, 2008 at 11:01 AM, Scott Whitney <swhitney(at)journyx(dot)com>

> Fully agreed, and it's just a concept at the moment. After I have a
> prototype standby working next week in the first place, we'll be discussing
> those very merits.
> A 2nd question: Is it possible to have 2 standby servers with a single
> master duplicating to standby1 (at my coloc), and standby2 (at my office)?
> Assume no auto-failover.
> -----Original Message-----
> From: Montaseri [mailto:montaseri(at)gmail(dot)com]
> Sent: Jun 26, 2008 12:51 PM
> To: Simon Riggs
> Cc: Scott Whitney; pgsql-admin(at)postgresql(dot)org
> Subject: Re: [ADMIN] Warm standby server
> I am not so sure of this arrangement's mertis
> From HA (High Availability) point of view, the host/server is a single
> point
> of failure which will bring your entire infrastructure down if any of the
> server hardware components fail.
> From Performance point of view, you have increased the load on your server
> by 3 folds as all instances would be using your I/O bandwidth to write to
> secondary storage
> Given $300 to $400 price of headless servers these days, its much
> economical
> to split the workload on three boxes
> Cheers
> Medi
> On Thu, Jun 26, 2008 at 9:06 AM, Simon Riggs <simon(at)2ndquadrant(dot)com>
> wrote:
>        On Thu, 2008-06-26 at 10:19 -0500, Scott Whitney wrote:
>        > I've got 3 different database servers (db01, db02 and db03).
>        >
>        > I would like to have a WAL standby server that replays logs for
> all 3 in
>        > case one goes down, so I can promote that particular server.
>        >
>        > Can I do this by installing 3 separate postmasters on this
> machine?
>        > Obviously, if 2 went down at the same time, I'd have to do some
> magic to
>        > bring up another machine, but I'm not sure that's a concern.
>        Yes, that will work.
>        --
>         Simon Riggs 
>         PostgreSQL Training, Services and Support
>        --
>        Sent via pgsql-admin mailing list (pgsql-admin(at)postgresql(dot)org)
>        To make changes to your subscription:

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