speed of a ciss-based pseudo-disk

[Scott Long]

Mikhail Teterin wrote:
> Hello!
> 
> I have two U320 SCSI drives in a RAID-0 combination attached to HP's
> Smart Array 642. This is the dmesg:
> 
> ciss0: <HP Smart Array 642> port 0xb800-0xb8ff mem 
> 0xfea80000-0xfeabffff,0xfeafe000-0xfeafffff irq 29 at device 1.0 on pci4
> da0 at ciss0 bus 0 target 0 lun 0
> da0: <COMPAQ RAID 0  VOLUME OK> Fixed Direct Access SCSI-0 device 
> da0: 135.168MB/s transfers
> da0: 69419MB (142171680 512 byte sectors: 255H 32S/T 17423C)
> 
> Why is the reported speed only 135.168MB/s? All equipment is U320, so
> I'd expect the nominal speed of 320MB/s...
> 
> Should I try a different cable? Mine says U320, although the terminator
> is marked U160 -- but 135 is not even 160...
> 
> Any ideas?
> 

Just because a disk can communicate at Ultra320 doesn't mean that it can
sustain data at that rate.  Same goes for ATA133 and SATA150.  A typical
modern disk can sustain about 50-70MB/s, and that's it.  The fact that
you're getting 135MB/s on a 2 disk sets completely validates this.  What
Ultra320 gives you is the ability to sustain that 50-70MB/s on multiple
disks at once.

Scott