Tuning SYSVR3 (Esix Rev D) (LONG!)

[Conor P. Cahill]

In article <1990Dec4.031037.2718 at jwt.UUCP> john at jwt.UUCP (John Temples) writes:
>
>Is a cached hard disk controller a win in this situation?  If you
>flush a bunch of stuff to disk (but not so much as to overflow the
>controller's cache), does the system's response still feel "snappy"?

It depends upon the controller.  Some caching controllers have a write
through cache, so the updates actually go to the disk at the time
the os says so.  On these controllers you would see the same slow downs
when the buffers were flushed.

Other controllers (like the DPT Caching controller that we have here) cache
both reads and writes.  Performance is always snappy unless you try to 
read or write very large (i.e. greater than the cache size) number of
blocks.  The drawback to this is that if you suffer a power hit, the 
ordered writes that the kernel thought were complete, may not have been
completed and the writes that were complete may not have been done in
the right order.  Of course, if you can afford to get the caching 
controller you should be able to afford a UPS to protect you from this.

>What happens when you give the controller a read request while it's
>flushing its cache to the disk -- does the read take priority?  Or

Again, this depends upon the controller, but with the DPT the non-flush
activity has priority as long as there are enough non-dirty blocks in
the cache to satisify it.

>about 386 UNIX is that when you have two or more processes contending
>for the disk, the system comes to a near halt.  Will a cached
>controller help or eliminate this problem?

This depends upon the problem that you are seeing.  For example, if you
have two programs that are essentially reading the entire disk at the
same time, there is almost nothing that you can do.  If, however you
are talking about two compiles of rather large programs, the cached
controller should help alot.


-- 
Conor P. Cahill            (703)430-9247        Virtual Technologies, Inc.,
uunet!virtech!cpcahil                           46030 Manekin Plaza, Suite 160
                                                Sterling, VA 22170