Synchronous/Asynchronous and Disconnect/Connect?

[Michael Meissner]

On Tue, Oct 06, 1998 at 10:07:23PM +0100, Paul Haigh wrote:
> All,
> 
> Sorry If I'm asking a silly question but here goes anyway:
> 
> I'm a little confused about the difference between Sync and Async (and also 
> disconnect and connect).  I've read a lot of stuff about the changing of 
> harddisc jumper settings to alter the behaviour of the driver under Linux. 
>  (To either enable it or disable it)

A good SCSI FAQ is located at:

	http://www.ultranet.com/~gfield/gary/scsi.html

Basically Async is the original SCSI-1, Sync is newer and faster ('Fast' is the
20 Mb/s improvement, 'Ultra' is the 40 Mb/s bus speed).  Disconnect is for slow
devices not to hog the scsi bus.
 
> Am I missing a simple trick to speed up my discs?

No, what you are missing is the scsi bus speed (40 Mb/s) is the maximum
possible data movement for all devices connected to the bus (ie, you might be
able to reach 40 Mb/s if you have 3-4 cheetah's all tallking at the same time).
It's the guaranteed not to exceed speed, not the average speed you will see.
This by the way is the same FUD that the IDE world experiences (ie, no IDE disk
in the world talks at a continuous 33 Mb/s).

> I've got two older Sun Microsystems discs in my linux server (both 4.5Mb 
> SCA drives)

About 5 Mb/s is about what you would expect for a single disk of that era.  You
might get faster performance if you stripe them together.

> I only get around 5Mb/sec on these drives, which I would say was less than 
> I was expecting.  Is the sync/async thing slowing me down?

No, your disks are sync/wide anyways.  They are not Ultra drives (ie, 40 Mb/s),
merely 'Fast' drives (ie, 20 Mb/s with wide transfers).

-- 
Michael Meissner, Cygnus Solutions (Massachusetts office)
4th floor, 955 Massachusetts Avenue, Cambridge, MA 02139, USA
meissner at cygnus.com,	617-354-5416 (office),	617-354-7161 (fax)

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