SMARTDrive: Maximum Cache Size (95531)

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:

Use the following values:
ElementSize = 8192 bytes
SegmentSpace = 38400 bytes
ElementDescriptor = 12 bytes

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.