On Thu, Apr 18, 2002 at 02:39:17PM -0600, Greg Oster wrote:
> Jeff Rizzo writes:
> > (scenario #2)
> > The second thing I tried was to set up a RAID5 of the four disks.  Since
> > I only have four disks total, I wanted to set up the raid5 with three
> > disks and a "dummy", so I could create a degraded raid5 that I could 
> > add the fourth disk to once I copied the system to the new raid5.  I have
> > gotten this to work successfully with RAID1 before, but raidframe would
> > not let me configure (even with -C) the 4-disk raid5 with only 3 disks.
> > Is this expected?  
> 
> No.. You should be able to create a 4-disk RAID 5 set with just 3 disks and a 
> 'dummy'.  Note that the 'dummy' disk must have a valid entry in /dev
> (but the device doesn't have to physically exist).  

OK, I think I chose a device which didn't have a file in /dev, so that 
was most likely the problem.

> > Any feedback/input would be most appreciated... I've now got it running
> > a 3-disk raid5, but the write performance is so bad I'm considering
> > other options. 
> 
> With a little tweaking, you can probably improve the write performance a fair 
> bit... e.g. what is the block/frag size on the filesystem, and what is the 
> stripe width that you've selected?  With some playing around, you should be 
> able to bump up the perf numbers a fair bit... 

OK, I'm doing some of that tweaking now, playing with different 
block/frag sizes and stripe widths.  One issue that's come up is
that I'm not entirely sure what the accepted way of recreating a 
raid set with different params is... when I change something in the
conf file (like stripe width), and reconfigure the raid set, it
claims the parity's still good, which seems counterintuitive to
me.  Perhaps I need to erase the component labels?  Is there
any way to do this short of dd'ing the whole disk with zeros?

Also, I know that the tradeoffs of stripe width and block size are
the subject of much research, but is there any site (or other
reference) which has some discussion of the tradeoffs expected?  I've
seen your raidframe webpage, and that's helped get me started.

One more thing - when I was asking about configuring the 4-disk
raid5 on irc, someone mentioned that 4 disks might not be a good
choice for raid5 because of how the stripes land on the disk, and
referred to some discussion on one of the lists.  I have not
been able to find this discussion, and wondered if there's anything
to the assertion?


Thanks a ton,
+j

-- 
Jeff Rizzo                                         http://boogers.sf.ca.us/~riz