Re: Clustering & Load Balancing & Replication - Mailing list pgsql-general

Thanks Chris,
I see you a core member of Slony team and a replication guru so I'll look
into it.
I'm not slamming Slony I think its probably the right tool for type of work
your company Afilias does. Just wish you would make an official Windows
version of Slony as well.
Anyway thanks for the education, and I think it would be a good thing if
your site on replication, was also listed on Postgresql... good research.
Merry Xmas

----- Original Message -----
From: "Christopher Browne" <cbbrowne@acm.org>
To: <pgsql-general@postgresql.org>
Sent: Sunday, December 24, 2006 4:23 AM
Subject: Re: [GENERAL] Clustering & Load Balancing & Replication


> Centuries ago, Nostradamus foresaw when org@kewlstuff.co.za would write:
>> Suggest you download my little application and read the documentation,
>> you'll see its very different, maybe even interesting.
>> Maybe they should change that to.... Postgres DOES HAVE a free
>> multi-master
>> replication system :)
>
> It isn't systematically usable as such, without a whole lot of
> end-user assembly.
>
>> One comment they make.... "Heavy write activity can cause excessive
>> locking,
>> leading to poor performance. In fact, write performance is often worse
>> than
>> that of a single server. Read requests can be sent to any server."
>> I'm not sure I agree with that... or maybe MVCC is just fantastic.... I
>> tested it.
>> The 2 phase commit locking is definitely happening at record level, so
>> only
>> if the multimasters all hit the same record is there the potential for
>> lock
>> conflict.
>> Why will dB's being randomly used, hit the same records, I think its a
>> low
>> probability to begin with?
>
> That's only true if you are certain that the update pattern is NOT
> involving a shared set of records.  IN GENERAL, heavy write activity
> can cause locking to become mighty expensive, which is certainly a
> true statement.
>
>> Not happy with that, I wrote a multithreaded routine and got them to all
>> smack the same record, it NEVER ROLLED BACK, and if there is performance
>> degradation, I didnt notice it... again probably a testament to the MVCC
>> design.
>
> It seems likely to me that this requires some careful validation of
> testing.
>
> An effect we see is that if a set of transactions are "fighting" over
> a single "balance" record, they will essentially serialize over that.
>
> On a system with a single CPU, it is not obvious that you'll see a
> degradation there because, since you only have the single CPU, it
> would be serializing the activity anyways.
>
> Try it out on an 8-way SMP system and you may see things differently.
>
>> In any event if you look at the documentation, you'll see SPAR is not
>> multimaster or nothing. Can use say one server in an office and another
>> to
>> pump data to a remote web site... not sure if you would even call that
>> multimaster, thats the point, I'm not sure SPAR fits any pure theory
>> category.
>
> There are a few tests I could throw at it that tend to challenge
> replication systems vis-a-vis "fidelity of results."  I otta see if I
> can find them in a readily deployable form.
>
> There are two notable anomalies which have been known to break
> replication systems:
>
> 1.  Nondeterministic updates:
>
> For instance, functions that are nondeterministic:
>
>  insert into rtable values (random(), now());
>
> Or result sets that are nondeterministic:
>
>  insert into rtable2 (select * from mytable where some_attr='foo'
>     order by random() limit 5);   -- Where there are 25 records with
> some_attr='foo'
>
> 2.  Value swapping:
>
> Consider the table:
>
> create table t1 (mk integer primary key, val text unique not null);
>
> insert into t1 (mk, val) values (1, 'chris');
> insert into t1 (mk, val) values (2, 'dave');
> insert into t1 (mk, val) values (3, 'brad');
>
> begin;
> update t1 set mk = 99 where mk = 1;
> update t1 set mk = 1 where mk = 3;
> update t1 set mk = 3 where mk = 99;
> commit;
>
> Is there a condition where a pause somewhere in there will cause
> replication to break?  Note that there have been replication systems
> (erServer) that this set of updates can, intermittently, cause to fall
> over.
> --
> let name="cbbrowne" and tld="linuxfinances.info" in String.concat "@"
> [name;tld];;
> http://cbbrowne.com/info/slony.html
> "Feel free to  contact me (flames about my english  and the useless of
> this driver will be redirected to /dev/null, oh no, it's full...)"
> -- Michael Beck, describing the PC-speaker sound device
>
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