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Re: Warm standby server

From: "Scott Whitney" <swhitney(at)journyx(dot)com>
To: "'Montaseri'" <montaseri(at)gmail(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 18:28:31
Message-ID: 20080626182511.6272F3E219D@mail.int.journyx.com (view raw or flat)
Thread:
Lists: pgsql-admin
I was thinking along the same lines. However, having 2 standby servers is a
must for us. If my colocation center gets razed to the ground, I need to
have a standby server elsewhere. Having one at the colocation center is for
soft/hardware disasters on any primary. Having one at my office is for
facility disasters at the colocation center.

I envision it working like this:

The archive command pseudocode will be:

rsync <mywalfile> someone(at)abox

The standby slave servers will then continually rsync that directory while
in recovery mode.

Reading the docs, it looks as if this should work.

It's also my understanding that the archive command happens asynchronously,
so it shouldn't affect the overall performance of the primary. Regardless,
my proposed solution should handle such a scenario. 

-----Original Message-----
From: Montaseri [mailto:montaseri(at)gmail(dot)com] 
Sent: Jun 26, 2008 1:21 PM
To: Scott Whitney
Cc: Simon Riggs; pgsql-admin(at)postgresql(dot)org
Subject: Re: [ADMIN] Warm standby server

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. 

Thanks
Medi


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


	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           www.2ndQuadrant.com
	        PostgreSQL Training, Services and Support
	
	
	
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