[Linux-cluster] Using GFS on Compact Flash

[Robert Peterson]

Lin Shen (lshen) wrote:
> Does it make sense to use  GFS (local or cluster mode) on Compact Flash?
> Will it greatly reduce the life expectancy of the Compact Flash compared
> to using a local file system? The rational behind this is that GFS will
> issue way more writes to the disk for its internal operations ( such as
> dlm locking etc).
> 
> Lin   
Hi Lin,

Here are my thoughts:

I'm not aware of anyone using compact flash with GFS.
GFS has no wear-leveling, so life expectancy might be an issue.
The same goes for most file systems except those specifically
written for CF, like jffs2.

The file and directory data isn't a concern: Linux page cache should 
manage the data buffers normally.  The cluster locks and glocks are 
also not a concern, since everything is managed in memory.  However, 
the GFS Resource Groups (RGs) (Not to be confused with rgmanager's 
resource groups) are a bigger concern.  Part of the RGs have bitmaps
of blocks to indicate which blocks are allocated and that may be rewritten
many times as blocks are allocated and released.  However,
I haven't studied how often these are actually written back to the media.

Another concern are the journal areas, which are being written over and over.
They aren't as bad as the RGs, though, because the journals have
lots of space, and therefore it's not always the same block getting
written over and over.  The journals may be forced to disk more though,
because if a node crashes for whatever reason, the other nodes need access
to the journals to replay the node's data to ensure file system integrity.

Regards,

Bob Peterson
Red Hat Cluster Suite