[Linux-cluster] Question about GFS2 journal size

[Bob Peterson]

----- Original Message -----
| The default journal size for gfs2 is 128 megs. We have a 8 node
| cluster, so
| I create 10, just in case. So that yields to roughly 1.3 gig the size
| of an
| empty filesystem.
| 
| For some applications, I wish to create smaller file systems, like
| bellow 1
| gig. So to have 1.3 gig of journal for a 500 megs FS is like
| overkill.
| What's the use of the journal and can I lower down to a more
| proprotionate
| size for my application, like 16 or 32 meg each ?

Hi,

For GFS2 journals, you don't need to create extras like GFS1.
You only need one journal _per_mounting_node.  So your cluster
can be 32 nodes, but if only two of those nodes are going to
mount the file system, you only need 2 journals. So the question
is how many of your 8 nodes are going to mount?
If all 8 need to mount it simultaneously, then create 8 journals.
With GFS2, you can always easily add more journals with gfs2_jadd
(unlike GFS1).

Now as to journal size: The biggest factor to consider here is
performance. The reason you want big journals is so that the
processes writing metadata to the file system don't have to pause
and wait for journal flushing very often, which forces the journal
to be synced to the media, which is slow. IIRC, the minimum journal
size is 32 (MB), so 16 isn't an option. You probably wouldn't want
16MB anyway, because it would impact performance, as explained above.

So it depends on how you're planning to use the file system.

Regards,

Bob Peterson
Red Hat File Systems