Q95531: SMARTDrive: Maximum Cache Size

Article: Q95531
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:3.1,3.11
Operating System(s): 
Keyword(s): 
Last Modified: 01-OCT-1999

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The information in this article applies to:

- Microsoft Windows versions 3.1, 3.11 
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SUMMARY
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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

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

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