[comp.sys.dec] Strange Disk Performance Help!

sritacco@hpdmd48.boi.hp.com (Steve Ritacco) (09/11/90)

Hi,

After reading the posting about strange disk performance on the DECstation
I got pretty work up about the situation on my system.

I'm running a DECstation 2100 (UWS 2.1) with two MAXTOR LXT-213sy disks.

Running "time dd if=/vmunix of=/dev/null bs=8k" I'm only seeing about 
400K / second.  Is that all there is?  I got excited when I read the last
disk performance posting which mentioned SCSI_TRYSYNC which I didn't know
about.  My disks have a 32k read ahead cache and will opperate in 
synchronous mode.  I made the SCSI_TRYSYNC change to scsi_data and
rebuilt my kernel, but it didn't help.  Does anyone know what the
problem could be.  When I hear that new RZ55s are doing 1.5M/second on
the DECstations I get seriously upset.  Does anyone know how I can tell
if the disk is actually managing to run in sync mode.  Can anyone help?

If that is all the disk performance I'm going to get I would just like
to know so I can learn to live with it.  If UWS 4.0 will save me, I'd like
to know that too.  


HELP !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

lp@decvax.dec.com (Larry Palmer ) (09/18/90)

-------

The difference in performance (dec RZ55 vs other disks) comes from
the enabling of a read ahead cache on the disk drive. This greatly
improves both raw and file system performance. Issue the command

        rzdisk -g current /dev/rrz?a

and page through the output. Page code 8 (cacheing parameters) on
the RZ55 will show that the read cache is enabled (on greater than
rev 700 firmware RZ55's). Some other vendors also allow read caching
to be enabled on their disks but most do NOT enable it by default.
You might try rzdisk on an unused (and unimportant) drive and
see if the drive produces anything rzdisk understand (each vendor
seems to want to put things is different format pages). The -g
option wont hurt your disk but others might so be careful.

The TRYSYNC field is probably working if it has been set and
the drive understand how to do synchronous transfers.


---   Larry Palmer		 Ultrix Networking/AD
---   Digital Equipment Corporation
---   Nashua, New Hampster
---   ... One of the fathers of PMAX ...

iglesias@orion.oac.uci.edu (Mike Iglesias) (09/19/90)

A rep from Maxtor came out and turned off the read cache disable bit
in the Maxtor XT-8380s drive we had, and it improved the read speed
(for a dd of the raw a partition to /dev/null) quite a bit - 394kb/sec
vs. 1.2mb/sec!  Using rzdisk -c ask /dev/rrz?c should work - I didn't
actually update the disk, but it did ask me about changing the read
cache disable bit.



Mike Iglesias
University of California, Irvine
Internet:    iglesias@orion.oac.uci.edu
BITNET:      iglesias@uci
uucp:        ...!ucbvax!ucivax!iglesias