On Tue, May 23, 2006 at 11:04:44AM -0400, Jon LaBadie wrote:
> Recently I had a look at amplot results for my new vtape setup.
> One thing it showed was that for 2/3 of the time, only one of the
> default four dumpers was active.

This is a good point.  amplot is awesome for checking out what kind of
stuff is slowing down your backups!  Also check the output at the end
of amstatus when a run is finished.  It'll give you a summary of the
same information.  But there's nothing like a cool graph!

As far as the original poster's question: I think you should try it
out.  Whether it's a performance win or loss is going to depend
heavily on how the data has ended up across those disks.

Your RAID5 performance is always dominated by the time it takes to
seek for data.  If all n disks can just just stream for a while, you
get full streaming performance from the disks.  But if even one of
them needs to seek to find its blocks, you're going to have to wait
until that disk finishes.

This makes me think that in most cases, dumping a big RAID5 in
parallell would hurt performance.  However, if your array is old, it
may be highly fragmented.  The extra I/O requests might be smoothed
over by an elevator algorith somewhere, and you might fit more data
into the same time...

I'd say it calls for an experiment.

--
Ross Vandegrift
ross AT kallisti DOT us

"The good Christian should beware of mathematicians, and all those who
make empty prophecies. The danger already exists that the mathematicians
have made a covenant with the devil to darken the spirit and to confine
man in the bonds of Hell."
        --St. Augustine, De Genesi ad Litteram, Book II, xviii, 37