maria-developers team mailing list archive

[Alex Yurchenko <alexey.yurchenko@xxxxxxxxxxxxx>]

On 2013-11-15 19:34, Pavel Ivanov wrote:
> On Fri, Nov 15, 2013 at 1:28 AM, Alex Yurchenko
> <alexey.yurchenko@xxxxxxxxxxxxx> wrote:
> > Please pardon this arrogant interruption of your discussion and 
> > shameless
> > self-promotion, but I just could not help noticing that Galera 
> > replication
> > was designed specifically with these goals in mind. And it does seem 
> > to
> > achieve them better than semi-sync plugin. Have you considered Galera? 
> > What
> > makes you prefer semi-sync over Galera, if I may ask?
>
> To be honest I never looked at how Galera works before. I've looked at
> it now and I don't see how it can fit with us. The major disadvantages
> I immediately see:
> 1. Synchronous replication. That means client must wait while
> transaction is applied on all nodes which is unacceptably big latency
> of each transaction. And what if there's a network blip and some node
> becomes inaccessible? All writes will just freeze? I see the statement
> that "failed nodes automatically excluded from the cluster", but to do
> that cluster must wait for some timeout in case it's indeed a network
> blip and node will "quickly" reconnect. And every client must wait for
> cluster to decide what happened with that one node.
> 2. Let's say node fell out of the cluster for 5 minutes and then
> reconnected. I guess it will be treated as "new node", it will
> generate state transfer and the node will start downloading the whole
> database? And while it's trying to download say 500GB of data files
> all other nodes (or maybe just donor?) won't be able to change those
> files locally and thus will blow up its memory consumption. That means
> they could quickly run out of memory and "new node" won't be able to
> finish its "initialization"...
> 3. It looks like there's strong asymmetry in starting cluster nodes --
> the first one should be started with empty wsrep_cluster_address and
> all others should be started with the address of the first node. So I
> can't start all nodes uniformly and then issue some commands to
> connect them to each other. That's bad.
> 4. What's the transition path? How do I upgrade MySQL/MariaDB
> replicating using usual replication to Galera? It looks like there's
> no such path and the solution is stop the world using regular
> replication and restart it using Galera. Sorry I can't do that with
> our production systems.
>
> I believe these problems are severe enough for us, so that we can't
> work with Galera.

Pavel, you seem to be terribly mistaken on almost all accounts:

1. *Replication* (i.e. data buffer copying) is indeed synchronous. But 
nobody said that commit is. What Galera does is very similar to 
semi-sync, except that it does it technically better. I would not dare 
to suggest Galera replication if I didn't believe it to be superior to 
semi-sync in every respect. As an example here's an independent 
comparison of Galera vs. semi-sync performance: 
http://linsenraum.de/erkules/2011/06/momentum-galera.html. In fact, 
majority of Galera users migrated from the regular *asynchronous* MySQL 
replication, which I think is a testimony to Galera performance.

2. Node reconnecting to cluster will normally receive only events that 
it missed while being disconnected.

3. You are partially right about it, but isn't it much different from 
regular MySQL replication where you first need to set up master and then 
connect slaves (even if you have physically launched the servers at the 
same time). Yet, Galera nodes can be started simultaneously and then 
joined together by setting wsrep_cluster_address from mysql client 
connection. This is not advertised method, because in that case state 
snapshot transfer can be done only by mysqldump. If you set the address 
in advance, rsync or xtrabackup can be used to provision the fresh node.

4. Every Galera node can perfectly work as either master or slave to 
native MySQL replication. So migration path is quite clear.

It is very sad that you happen to have such gross misconceptions about 
Galera. If those were true, how would MariaDB Galera Cluster get paying 
customers? May be my reply will convince you to have a second look at 
it. (In addition to the above Galera is fully multi-master, does 
parallel applying and works great in WAN)

Kind regards,
Alex

> Pavel

--
Alexey Yurchenko,
Codership Oy, www.codership.com
Skype: alexey.yurchenko, Phone: +358-400-516-011