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

----- 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
| 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


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.


Bob Peterson
Red Hat File Systems

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