Fwd: Disk scheduling activity...

[Willem Jan Withagen]

Hi,

Just a point of information or curiosity, and I don't think/know if it
is any problem...

I have the raidz array with 8 disks, which I'm using to backup to.
It is configured
	4 disks on a mvs controller
	4 disks on an Areca controller (JBODs with battery)
	Both controllers are on a PCI-E slot

dT: 1.366s  w: 1.000s
  L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
     6    127      4      2    0.1    122   6481   18.8   56.5| da0
     4    118      0      0    0.0    116   6646   19.6   56.5| da1
     0    124      2      1    0.1    120   6650   19.0   56.6| da2
     5    117      1      1    0.1    114   6726   20.0   56.5| da3
     4    121      1      1    0.2    118   6895   25.7   70.6| ada0
     2    115      1      0   30.0    113   7051   22.1   67.7| ada1
     0    113      3      1   11.6    108   7057   26.3   73.7| ada2
     3    105      1      0   10.6    103   6728   24.1   66.9| ada3

Most of the time the source just fully loads the pipe and sends 1Gbit/s.

When that happens I see this alternating pattern of writing either to
the 4 mvs disks, or writing to the Areca disks.
But almost never are all disk accesses at the same time.
And really never, never is there a mix of writing between the controller
sets.

I even made a little movie of it. :)
	http://www.tegenbosch28.nl./FreeBSD/disk-activity.mov

What is in the disk scheduling that makes it this alternating pattern?
And why are not both controllers writing (flushing?) at the same time.

Changing the 4 ada?/mvs ports to ports on the motherboard behind a 
port-multiplier starts increasing the write time per disk, and thus 
increases the percentage that the disks are busy. So it is better to 
hang them off the mvs controller.

--WjW