Slow RA 90 disks on DECsystem 5XXX

[Alan's Home for Wayward Notes File.]

In article <1990Dec12.194341.18284 at bernina.ethz.ch>, karrer at bernina.ethz.ch (Andreas Karrer) writes:
> We have 6 DEC RA 90 disks, three on a DECsystem 5840 (BI bus, KDB50 controller)

	I have 8 RA90s scattered between two KDB50s and an HSC70
	on a VAX 8800.

> and 3 on a DECsystem 5400 (Q-Bus, KDA50 controller).

	I don't anything with a KDA50.  The controller itself
	is about the same speed as the KDB50.  Any differences
	in a similar configuration are due to the bus and CPU.

> 
> We are not impressed by their performance. Some comparisons:

	For the number presented neither am I.  Let's find out
	why...
> 
>   Copy a 10 M file between two partitions on different disks
>   (load average was 0.1 and less in all cases

	Here's my table.  The file sizes varied because that's
	what was BIG and handy on that disk.  Where it was
	obvious the was sitting in the buffer cache, I note
	that.  One test I ran twice and pretty close to the
	same result (2).

File size		Configuration			 Bandwidth
10,672 KB	RA90 to RA90 on same KDB50.		381 KB/sec.
46,576 KB	RA90 to RA90 on same HSC70		375 KB/sec.
77,144 KB	RA90 to RA90 different KDB50s.		756 KB/sec.

10,672 KB	RA90 on KDB50 to /dev/null		820 KB/sec.
46,576 KB	RA90 on HSC70 to /dev/null		716 KB/sec.

10,672 KB	Buffer cache to RA90 on KDB50.		711 KB/sec.
10,672 KB	Buffer cache to RA90 on HSC70.		711 KB/sec (2).

46,576 KB	RA90 on HSC70 to RA90 on KDB50		529 KB/sec.
46,576 KB	RA90 on KDB50 to RA90 on HSC70		665 KB/sec.

> 
>  questions:
> - Am I doing something wrong here? Yeah I tried upping the bufcache to 25 %,
>   didn't help any. Please no benchmark flames...

	Are the disks on the SAME controller?  Yes, disk to disk on
	the same KDB50 is relatively slow.  If you're going to be
	doing lots of disk to disk traffic consider splitting the 
	disks between multiple disk controllers.  Also consider a
	KDM70.

> - does a HSC help?

	It depends.  If the disks are on the same requestor (like mine)
	you have the same problem as the same KDB50 to KDB50 case.  If
	the disks are on different requestor it might be better.  For
	a small number of disks you're probably better off with a few 
	KDB50s or KDM70s.  After 32 or more about the only way you can
	get there is multiple HSCs.

	Please note that KDM70 also supports TA tape drives.

> - are there third party disks (faster, cheaper, or both) for the DS 5xxx?

	I don't know.
> 
> +-----------
>   Andi Karrer, Communication Systems, ETH Zuerich, Switzerland
>   karrer at ks.id.ethz.ch

-- 
Alan Rollow				alan at nabeth.enet.dec.com