Re: MirrorMode VS load balancing

[Hideo NAKAMITSU <nomo@bluecoara.net>]

Hi, Brett,

On Sun, 28 Jun 2009 13:36:03 +1000
"Brett @Google" <brett.maxfield@gmail.com> wrote:

> > but on some cases(including my environment), UPDATE happens 3-10 times in a
> > day.
> > it's much less than SEARCH.
> > in this case, I think we can just put loadbalancer, and enable "load
> > balancing" and "redundancy"
> > (I understand UPDATE isn't load balanced.)
> 
> 
> Update is not load balanced, but it could be. You could have a "read only"
> balance rule, and a "read write" balance rule, either on sererate ip or
> seperate port. They would both point to the same servers, but the balance
> rules would act differently based on the selectioj of ip / port.
> 
> If you are able to distinguish between read only and read write
> applications, then you could configure your read only applications for
> "round robin", and your read write applications for "sorry server" ie.
> traffic only hits the secondary if the primary is down.
> 
> Ie. a typical use is for mapping email adreesses, in this case the mailers
> are only ever frequent read, and whatever puts the emails in are seldom
> write (by comparison). In this case mailers need high speed round robin for
> load sharing, updating application only need's it's writes to be accurately
> stored (extreme speed is not such a priority)
> 
> Alternately if you have one application that does high frequency read and
> write, you could always put a ldap proxy in front and try to divert the
> writes toward the "read write" balance, and the read only queries toward the
> "read only" balance, via use of a slave syncrepl and updatedn.

I understand your case. what I wanted to do is this.

- I have L4 load balancer
- want to do load balancing(for search) & HA
- no frequency "writes"
- simple as possible

to separate LB configuration(port,host), or building LDAP proxy maybe easy, but complicated work.
if I can build ldap servers as following, I believe it's simpler.

--------------------------------------------------------
   CLIENT
     |
    SLB(RoundRobin)
     |
  +-----+
 SRV1  SRV2

* MirrorMode is enabled on SRV1 and SRV2
--------------------------------------------------------

the problem is as Howard said, "writes period"
If there are too much "writes" in every seconds, I understand I shouldn't choose this.
but if it is 10writes/min, it must be safe.

and how about 1write/sec, 10writes/sec, 50writes/sec, 100writes/sec...
I want to know this threshold like,

- if writes happen 0-5/sec, it's 99% safe
  (at the worst unlucky case, only 2writes at the same time may break something)
- 6-50/sec, may safe or not.
- more than 50/sec, danger.

Regards,