--On Freitag, September 07, 2007 20:00:16 +0100 Simon Riggs
<simon@2ndquadrant.com> wrote:

> On Fri, 2007-09-07 at 11:37 +0200, Maila Fatticcioni wrote:
>
>> protocol C;
>
> Try protocol B instead.

But that would have an impact on transaction safety, wouldn't it? It will
return immediately after reaching the remote buffer cache and you can't be
sure your data hits the remote disk.
It's a while ago i've played with such a setup, but it could be worth to
play around with max_buffers, al-extends, snd-bufsize. Oh and i think
Maila's 'rate' setting is too high: i've found rate settings
counterproductive when set too high (try a value slightly above your max
bandwidth of your connection). But i second Heikki, you should take care on
your disk setup as well.

--
Thanks

Bernd

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On Fri, 2007-09-07 at 23:54 +0200, Bernd Helmle wrote:
> --On Freitag, September 07, 2007 20:00:16 +0100 Simon Riggs
> <simon@2ndquadrant.com> wrote:
>
> > On Fri, 2007-09-07 at 11:37 +0200, Maila Fatticcioni wrote:
> >
> >> protocol C;
> >
> > Try protocol B instead.
>
> But that would have an impact on transaction safety, wouldn't it? It will
> return immediately after reaching the remote buffer cache and you can't be
> sure your data hits the remote disk.

You're right, but the distinction is a small one. What are the chances
of losing two independent servers within a few milliseconds of each
other?

If performance is an issue it is a particularly important distinction.

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com


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"Simon Riggs" <simon@2ndquadrant.com> writes:

> You're right, but the distinction is a small one. What are the chances
> of losing two independent servers within a few milliseconds of each
> other?

If they're on the same power bus?

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com

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Gregory Stark wrote:
> "Simon Riggs" <simon@2ndquadrant.com> writes:
>
>> You're right, but the distinction is a small one. What are the chances
>> of losing two independent servers within a few milliseconds of each
>> other?
>
> If they're on the same power bus?

That chance is minuscule or at least should be. Of course we are
assuming some level of conditioned power that is independent of the
power bus, e.g; a UPS.

Joshua D. Drake





- --

=== The PostgreSQL Company: Command Prompt, Inc. ===
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Joshua D. Drake wrote:
> Gregory Stark wrote:
>> "Simon Riggs" <simon@2ndquadrant.com> writes:
>
>>> You're right, but the distinction is a small one. What are the chances
>>> of losing two independent servers within a few milliseconds of each
>>> other?
>> If they're on the same power bus?
>
> That chance is minuscule or at least should be. Of course we are
> assuming some level of conditioned power that is independent of the
> power bus, e.g; a UPS.

how is that making it different in practise ? - if both are on the same
UPS they are affectively on the same power bus ...
If the UPS fails (or the generator is not kicking in which happens way
more often than people would believe) they could still fail at the very
same time ....


Stefan

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"Joshua D. Drake" <jd@commandprompt.com> writes:
> Gregory Stark wrote:
>> "Simon Riggs" <simon@2ndquadrant.com> writes:
>>> You're right, but the distinction is a small one. What are the chances
>>> of losing two independent servers within a few milliseconds of each
>>> other?
>>
>> If they're on the same power bus?

> That chance is minuscule or at least should be.

It seems a bit silly to be doing replication to a slave server that has
any common point of failure with the master.

However, it seems like the point here is not so much "can you recover
your data" as what a commit means. Do you want a commit reported to the
client to mean the data is safely down to disk in both places, or only
one?

regards, tom lane

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"Joshua D. Drake" <jd@commandprompt.com> writes:

> That chance is minuscule or at least should be. Of course we are
> assuming some level of conditioned power that is independent of the
> power bus, e.g; a UPS.

I find your faith in UPSes charmingly quaint.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com

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Stefan Kaltenbrunner wrote:
> Joshua D. Drake wrote:
>> Gregory Stark wrote:
>>> "Simon Riggs" <simon@2ndquadrant.com> writes:
>>>> You're right, but the distinction is a small one. What are the chances
>>>> of losing two independent servers within a few milliseconds of each
>>>> other?
>>> If they're on the same power bus?
>> That chance is minuscule or at least should be. Of course we are
>> assuming some level of conditioned power that is independent of the
>> power bus, e.g; a UPS.
>
> how is that making it different in practise ? - if both are on the same
> UPS they are affectively on the same power bus ...

Well I was thinking the bus that is in the wall. I would assume that
people were smart enough to have independent UPS systems for each server.

city power->line conditioning generator->panel->plug->UPS->server

wash, rinse repeat.

> If the UPS fails (or the generator is not kicking in which happens way
> more often than people would believe) they could still fail at the very
> same time ....

Sincerely,

Joshua D. Drake





>
>
> Stefan
>
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>


- --

=== The PostgreSQL Company: Command Prompt, Inc. ===
Sales/Support: +1.503.667.4564 24x7/Emergency: +1.800.492.2240
PostgreSQL solutions since 1997 http://www.commandprompt.com/
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Gregory Stark wrote:
> "Joshua D. Drake" <jd@commandprompt.com> writes:
>
>> That chance is minuscule or at least should be. Of course we are
>> assuming some level of conditioned power that is independent of the
>> power bus, e.g; a UPS.
>
> I find your faith in UPSes charmingly quaint.

It isn't my faith in a UPS. It is my real world knowledge.

Further I will exert what I already replied to Stefan:

city power->line conditioning generator->panel->plug->UPS->server

You would have to have lightning handed by God to your server to have a
total power failure without proper shutdown in the above scenario.

Sincerely,

Joshua D. Drake








- --

=== The PostgreSQL Company: Command Prompt, Inc. ===
Sales/Support: +1.503.667.4564 24x7/Emergency: +1.800.492.2240
PostgreSQL solutions since 1997 http://www.commandprompt.com/
UNIQUE NOT NULL
Donate to the PostgreSQL Project: http://www.postgresql.org/about/donate
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Joshua D. Drake wrote:
> Stefan Kaltenbrunner wrote:
>> Joshua D. Drake wrote:
>>> Gregory Stark wrote:
>>>> "Simon Riggs" <simon@2ndquadrant.com> writes:
>>>>> You're right, but the distinction is a small one. What are the chances
>>>>> of losing two independent servers within a few milliseconds of each
>>>>> other?
>>>> If they're on the same power bus?
>>> That chance is minuscule or at least should be. Of course we are
>>> assuming some level of conditioned power that is independent of the
>>> power bus, e.g; a UPS.
>> how is that making it different in practise ? - if both are on the same
>> UPS they are affectively on the same power bus ...
>
> Well I was thinking the bus that is in the wall. I would assume that
> people were smart enough to have independent UPS systems for each server.
>
> city power->line conditioning generator->panel->plug->UPS->server
>
> wash, rinse repeat.

the typical datacenter version of this is actually more like:

city power->UPS (with generator in parallel)->panel->plug

or

city power->flywheel->(maybe UPS)->panel->plug

it is not really that common to have say two different UPS feeds in your
rack (at least not for normal housing or the average corporate
datacenter) - mostly you get two feeds from different power distribution
panels (so different breakers) but that's about it.
Having a local UPS attached is usually not really that helpful either
because those have limited capacity need space and are an additional
thing that can (and will) fail.


Stefan

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