DOCUMENT:Q95531  01-OCT-1999  [win95x]
TITLE   :SMARTDrive: Maximum Cache Size
PRODUCT :Microsoft Windows 95.x Retail Product
PROD/VER:WINDOWS:3.1,3.11
OPER/SYS:
KEYWORDS:

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Windows versions 3.1, 3.11 
-------------------------------------------------------------------------------

SUMMARY
=======

The cache size of SMARTDrive is limited by one 64 kilobyte (K) segment (or the
largest available upper memory block if you are loading SMARTDrive high). In
this 64K segment, SMARTDrive stores the code for its device driver (10.5K), a
read ahead buffer (16K) and an element table that references each element block
in the cache. The maximum cache size is limited by available RAM and the size of
the element table which is, in turn, limited by the amount of space left in the
64K segment after SMARTDrive and the read ahead buffer load.

If you are using the default element size (8K), SMARTDrive's cache has a maximum
size of 25 megabytes (MB). However, if you have an upper memory block (UMB)
provider (EMM386, for example) installed and you prevent SMARTDrive from loading
high by using the /L switch with the LH command, the read ahead buffer splits
off from the 64K segment. This makes more room for the element table, increasing
the maximum cache size of SMARTDrive to 35.7 MB.

MORE INFORMATION
================

To determine the maximum cache size of SMARTDrive, use the following formula:

   CacheSize = ElementSize * (SegmentSpace / ElementDescriptor)

ElementSize, by default, is 8K or 8192 bytes and refers to the element block size
in the SMARTDrive cache. ElementDescriptor is the memory required to reference
each element in the cache and is 12 bytes if the element size is 8K or greater;
it is 10 bytes if the element size is less than 8K. SegmentSpace can be
determined by subtracting the size of the SMARTDrive device driver and the read
ahead buffer from 64K (or the largest available upper memory block):

   One 64K segment:                       65536 bytes
   SMARTDrive device driver (10.5K):    - 10752 bytes
   Read ahead buffer (16K):             - 16384 bytes
   --------------------------------------------------
   Remaining segment space:               38400 bytes

   Note: If you have a UMB provider installed and you use the /L switch with the
   LH command, the read ahead buffer loads separately from the 64K segment,
   increasing the remaining segment space to 54784 bytes or 53.5K.

For example: If you do not load a UMB provider and do not change the default
element size of 8K, you can determine the maximum SMARTDrive cache size by doing
the following:

1. Use the following values:

   ElementSize = 8192 bytes
   SegmentSpace = 38400 bytes
   ElementDescriptor = 12 bytes

2. Insert the values from Step 1 in the formula

   CacheSize = ElementSize * (SegmentSpace / ElementDescriptor)

   such that:

   CacheSize = 8192 * (38400 / 12) = 26214400 bytes = 25 MB

NOTE: Although you can specify a large cache size, Microsoft recommends a maximum
cache size of 2 MB for most situations. Few systems benefit from large cache
sizes; beyond 2 MB, cache performance does not increase significantly.

Additional query words: 3.10 3.11

======================================================================
Keywords          :  
Technology        : kbWin3xSearch kbZNotKeyword3 kbWin310 kbWin311
Version           : WINDOWS:3.1,3.11

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1999.