Centuries ago, Nostradamus foresaw when org@xxxxxxxxxxxxxxx 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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TIP 2: Don't 'kill -9' the postmaster
