QUOTING ARTISOFT BULLETIN:

The SHARE Command Explained


                          What is SHARE?
The SHARE.EXE program is included with DOS versions 3.0 and
higher. Although most DOS manuals don't give it much coverage,
understanding SHARE can be vital to the success of your network,
especially if you are running a multi-user database, or a similar
application that works with several files at a time.

SHARE gives applications an easy, well-defined way to keep users
from accessing the same files, or the same regions of files
simultaneously. Once SHARE has been run, an application can use it
to "lock" a files or region so that only one person at a time can
make changes. Most multi-user and network software packages use
SHARE to implement their file and record locking. This bulletin
will discuss how SHARE works and how it can affect your LANtastic
network.
                         How SHARE Works
SHARE maintains two tables in memory. The first table, the FILES
table, contains the complete pathname of each file that has been
opened, plus an internal file handle number and other housekeeping
information.  The second table, the LOCK table, contains a list of
internal file handle numbers and corresponding information on the
various areas of each file that are locked. SHARE checks these
tables whenever an application asks to open or use a file or a
region of a file, and lets the application know whether or not the
file or region is available.

SHARE uses at least one entry in each table for each file that is
opened. The more files your computer opens and locks, the more
space SHARE needs for its internal tables.  You can control the
size of SHARE's internal tables with two command line options, /F
and /L. The /F option controls the amount of space allocated for
the FILES table and the /L option controls the number of
simultaneous locks that SHARE will allow.  To help you figure out
exactly how much space you'll need, let's examine each parameter
in detail.

The /F Parameter
The /F parameter controls the size (in bytes) of the table that
SHARE reserves for file names and file handles. The syntax for
using the /F parameter is

SHARE /F:n
where n is any number from 0 to approximately 62,000 (by empirical
test). The default is /F:2048. SHARE stores the complete pathname
of each file, plus 11 bytes of file handle and housekeeping
information.  You can find the worst case space requirement by
multiplying the number of files in your CONFIG.SYS by 71 (60 bytes
for the worst case pathname + 11 bytes for other information).
For a system with FILES = 255 in its CONFIG.SYS, that means that
in the worst case, with all 255 files open, SHARE will require
over 18,100 bytes for the FILES table.

On a network server, you will need to allocate enough space for
all the files that will be opened by all the users on the network.
The default value is 2048 bytes -- enough to hold the information
for 66 files with paths averaging 20 characters, or about 28 files
in a worst case scenario.  If you know that paths on your machine
average more than 20 characters, or that you will be opening lots
of files, you should probably use the /F parameter to give SHARE
more space for its FILES table.

The /L Parameter
The /L parameter controls the number of simultaneous locks that
SHARE can handle. It's probably the biggest potential troublemaker
for network users. The syntax for the /L parameter is

SHARE /L:n
where n is any integer between 1 and approximately 3800 (again, by
empirical test). The default is /L:20 -- that is, 20 locks. On a
network like LANtastic, which can open 5100 files per server, it's
easy to see that 20 locks just isn't enough.

Opening a file on a server requires at least one lock. In
addition, most network programs use several more locks per file.
They lock individual records, and even individual fields within
records.  Multi-user databases especially can use lots of locks,
sometimes 10 or more per file.  On a network, with several users
opening each file, SHARE's default 20 locks can be used up almost
instantly.

To add to the confusion, application programs behave in a somewhat
unpredictable manner when SHARE runs out of locks. Some programs
correctly report the error, some simply report "Access Denied" or
"Sharing Violation", and some just lock up the computer.  The
bottom line is that on your servers, you should use the /L
parameter to increase the number of locks allowed.  The /L setting
should be at least the number of files you've specified in your
CONFIG.SYS or in the NET_MGR Server Startup Parameters option
(whichever is larger). If you're running a multi-user program that
uses lots of files, you should consider setting /L to at least
twice the number of open files allowed.