dual-ported raid

[Daniel Pittman]

Jeff Garlough <jeffg at ahpcrc.org> writes:

> I have a dual-ported raid controller which allows two computers to
> connect to the same ext3 filesystem. I never mount both systems
> read-write at the same time. What I would like to do is use one
> normally, and mount the second system read-only to perform backups and
> to rsync the filesystem to another filesystem. 

That will not work, full stop, ever, with ext3.  Find another solution.

If you did do this, envision:

On the master node, where read/write activities are going on, we have a
bunch of on-disk data and a bunch of meta-data in memory.  Things like
inode allocation tables, etc.

These get written out to disk every now and then, through the journal
and for other reasons, on whatever schedule the master node feels is
worthwhile.


Meanwhile, over on the slave node you mount the file system.  It reads
some meta-data into memory and keeps it there, for convenience.  

You start working on data -- and, meanwhile, over on the master we
update some of the meta-data that the slave has in memory.


Now, the slave doesn't know that was updated, so it keeps using that
in-memory data happily.  Except, then it needs to load some fresh data
from disk and, pow, huge inconsistency in the file system.


ext3 alone cannot do what you want.  You might get away with it if you
can take a snapshot of the (consistent) state on the master, then mount
that on the slave, but that probably isn't a great plan either.


I strongly suggest you investigate some other solution like, say, simply
running your backups on the master.  You will have the same resource use
in both cases, pretty much, unless your rsync process is very checksum
intensive...

Regards,
        Daniel
-- 
Digital Infrastructure Solutions -- making IT simple, stable and secure
Phone: 0401 155 707        email: contact at digital-infrastructure.com.au
                 http://digital-infrastructure.com.au/