TBBS 2.x - Hard Disk Caching and TBBS

Contact:   eSoft, Inc. (Makers of TBBS)
           15200 E. Girard Ave., Suite 3000
           Aurora, CO  80014
           (303) 699-6565      Voice
           (303) 699-6872      Fax
           (303) 699-8222      BBS
           support@esoft.com   E-Mail

HARD DISK CACHING AND TBBS
--------------------------

A hard disk cache (pronounced like "cash") is a method of buffering the 
reading of data from the hard disk.  All disk reads are stored in a "cache" 
located somewhere in memory, as well as supplied to DOS or the program reading 
the data.  If DOS or a program needs to read the same data again (a very 
common occurance in many situations) then the cache itself intercepts the disk 
read, and feeds DOS or the program the data from memory, instead of the disk.  
This increases the throughput and apparent performance of the system.  Some 
circumstances can yield more benefit than others. 

There are two types of hard disk caches.  

SOFTWARE CACHES use a memory-resident program or driver.  This scheme puts the 
cache itself somewhere in system memory - conventional, extended or expanded.  
This approach steals a small amount of the CPU power away from applications in 
order to manage the caching operation.  It also requires memory installed in 
the CPU itself, either on the motherboard or on an expansion card.  Memory 
reserved for the cache cannot be used by application programs.  

HARDWARE CACHES build the entire caching operation into the disk controller 
itself.  This scheme requires no system memory of any kind, and does not steal 
any CPU power away from applications; a microprocessor and memory are built 
onto the controller itself.

Software caches can be a problem in some circumstances, and we recommend 
using EXTREME CAUTION in their use.  TBBS has a small amount of internal 
buffering that acts as a cache, and is adequate in most cases.  As with all 
memory resident programs, disk caching software can cause very subtle 
conflicts and problems, particularly in a high use environment like that of 
TBBS.  If you feel you must use software disk caching, stick with highly rated 
software such as Super PC-Kwik or PC Tools Deluxe PC-CACHE.  

Also, make certain that you DISABLE the write caching operation, if your 
caching software supports it.  FAILURE TO DISABLE WRITE CACHING WILL CAUSE 
SEVERE FILE DAMAGE IN A TBBS ENVIRONMENT IN MANY CASES!  

Keep in mind that disk caching is not realistic in an XT or AT computer due to 
the memory architecture of these machines.

When using a 386, 486 or Pentium based computer, make sure that the cache 
memory itself is located in extended (XMS) memory, and not expanded (EMS) 
memory.  TBBS itself uses EMS memory, and having the cache located there co-
resident with TBBS exponentially increases the possibility of conflicts.

Although hardware disk caching is more costly than software caching, it is 
substantially more reliable since it is almost totally independent of the main 
system and any software applications.  

If you are operating in a LAN environment, and if you want to use software 
based disk caching, we recommend you use the cache supplied with most LAN 
operating systems.  Most normal software caches cannot cache a LAN drive, and 
are therefore only of partial value in a LAN environment.  In addition, normal 
software caches can sometimes cause problems in LAN environments, such as 
lock-ups or file corruption.

TBBS0016
Rev. 10/90

Revised 10/93
Copyright (C) 1994 eSoft, Inc., All Rights Reserved.  Permission granted
to distribute this file in its entirety, without modification, to any
interested party.  Any other use requires the written permission of
eSoft, Inc.

IMPORTANT:  The information herein is subject to change without notice.
Please call or write to confirm factual information of importance to you
or your organization.

