Linux Raid Problem?

[Thomas Tonino]

David Durham <david.durham at wcom.com> wrote:

 > Before I describe this problem in more detail, can you tell me if you
 > have ever heard of a problem with Linux and the PowerEdge 2400 series
 > where the disk write speed is rediculusly slow around 3Meg/s
 > This is linux Mandrake 8.0 out of the box and I've configured via the
 > BIOS, the 3 17 gig SCSI drives as a single 30 gig Raid 5 container.

On the Dell 2550 with RAID0+1 with the battery backed on board 
controller the write performance was much better, but still much lower 
than I would expect.

Then I switched to Linux software RAID (is handled by the Mandrake 
installer: in the disk formatting GUI, make partitions in the Linux 
Software RAID type, and you can add them to RAIDs and use the RAIDs 
again to make file systems. But I could not figure out to make RAID0+1 
here). Here performance was much better: higher than an individual drive.

This applies even for RAID5 writing. Tried that on an external FC-AL 
cabinet. The Qlogic FC driver in the kernel does not work well with 
RAID5, but the driver from teh Qlogic site does. In this case write 
performance was >30 MB/sec if I remember correctly.

There probably still are bugs in the RAID code however, and there is a 
nuisance: of the system is not shut down cleanly, it will do a resync 
after booting.

I also tried one of the AMI controllers. Bit better than on board, but 
not by much. And when testing hot unplugging of drives, this controller 
managed once to mark the 3 drives that were left in the system as bad, 
and the one replugged drive as the only working one. That was easy 
enough to fix in the BIOS.

But we will go with software RAID. The reason: it will only get better 
over time. I got it booting from first and second drive by dd'ing the 
boot partition from the first drive onto the second. When the first 
drive is then unplugged, the system boots painlessly from the second drive.


Thomas