[Linux-cluster] Need help regarding Sared storage with GFS2

[Kaloyan Kovachev]

On Tue, 01 Nov 2011 09:03:41 -0400 (EDT), Bob Peterson
<rpeterso at redhat.com> wrote:
> ----- Original Message -----
> | Which one is not true "I had used common storage" or "On both the
> | nodes
> | data is not in sync" - if it is a common storage the data is the
> | same?
> | 
> | if you are using GFS2 without a cluster and dlm locking i.e.
> | local_locking
> | then it is possible both to be true
> (snip) 
> | GFS2 has nothing to do with syncing the data between two storages -
> | if
> | that's what you are after, check DRBD
> | 
> | if you are using improperly configured GFS2 on a shared storage i.e.
> | without cluster and dlm it is no different than any other local
> | filesystem
> | and corruption is guaranteed on simultaneous use
> 
> Hi,
> 
> IMHO, the most important things to bear in mind here are:
> 
> (1) The job of GFS2 is to keep the file system _metadata_ consistent
>     between nodes in the cluster.
> (2) It does _not_ keep DATA within the files consistent within the
>     cluster: that's the job of the application.
> (3) If the application is not cluster-aware (i.e. one instance of
>     mysql doesn't know about another instance in the cluster) they
>     will trounce each other's updates, making the data inconsistent.
> (4) The general rule is: If two instances of an app can run on the
>     same computer, in general it will work properly without data
>     corruption. But if one computer is not allowed to run two
>     instances of the same app, in general it will not work properly.
> (5) With clustering you can essentially think of it this way: it
>     makes multiple computers run an app as if they were running
>     multiple instances on the same computer.  Almost like forcing
>     the app to run two instances on the same computer (although
>     that's not at all what really happens).  Multiple instances
>     on the same machine will use some kind of locking mechanism,
>     like posix locks, to maintain data integrity.
> (6) Many apps are written with clustering in mind and there
>     may be special "clustered" versions of apps, like mysql.
>     It's best to check with the app experts or clustering experts
>     or the cluster FAQ before implementing this kind of thing.
> 
> So bottom line: You can't run two copies of regular mysql on the
> same box (unless it's a special cluster-aware mysql) without conflicts
> so you can't run two copies of regular mysql in a cluster without
> data corruption, because they are not cluster-aware.
> 

I agree with all said, but it is possible to run more than one instance of
regular mysql on the same box. I run 3 (slave of master 1, slave of master
2 and combined RO export) instances (on the same machine), using the same
data without problems, but you need to define 'external-locking' which
slows them down

running two instances in a cluster from shared storage is possible, but
much slower and not a solution.

> Regards,
> 
> Bob Peterson
> Red Hat File Systems
> 
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