SMARTDrive 4.0 Design Overview

• Microsoft Windows versions 3.1, 3.11
• Microsoft Windows for Workgroups version 3.1

SUMMARY

SMARTDrive version 4.0 is a utility included with Microsoft Windows and Microsoft Windows for Workgroups.

SMARTDrive 4.0 is designed as block-oriented disk cache. It hooks into the system at the MS-DOS device driver level, rather than at the ROM BIOS INT13 level. Each block device driver on the MS-DOS device driver chain is "front- ended" by SMARTDrive 4.0, which provides the caching.

SMARTDrive version 4.0 does not cache a temporary or permanent swapfile. Although SMARTDrive 4.0 is designed as a block-oriented disk cache and hooks into the system at the MS-DOS device driver level, SMARTDrive includes code which determines whether what is being cached is part of the swapfile. If SMARTDrive determines that disk access was made to access the swapfile, SMARTDrive. does not cache that data.

MORE INFORMATION

SMARTDrive 4.0 calls the MS-DOS level device driver to perform any needed physical disk I/O. This design yields the following benefits:

1. It is independent of the INT13 interface. Many device drivers do not use the INT13 interface. This means that these devices can be cached where INT13-level caches cannot. Examples are Bernoulli drives, some hard cards, and many SCSI and WORM drives. SMARTDrive 4.0 can cache any disk controller that uses an MS-DOS block device driver.



2. It is independent of disk geometry. Some disk managers and disk controllers use a disk-geometry mapping scheme that causes the logical geometry (that is, what MS-DOS sees) to be different from the physical geometry. Examples are many PS/2 systems, Ontrack's Disk Manager, and several disk controllers.
   
   INT13-based caches are sensitive to this difference and often have problems. For example, some disk-management software actually change the ROM BIOS specified disk geometry on the fly and thus confuse INT13-based caches.
   
   There usually is an interface to determine the true geometry, but this requires detection of the specific disk manager driver and generally complicates the disk cache software. Often, logical tracks actually cross physical track boundaries, which then cause track caches to incur performance penalties (inter-track seeks and rotational latencies).
   
   Also, to get around the ROM BIOS 1024 cylinder limitation, disk managers and controllers "fold" multiple tracks into 1 logical track. Doing this yields the above problem as well as forcing track caches to have a very large track buffer. In some cases, this cache is as large as 31.5K and must reside in low memory. The design of SMARTDrive 4.0 eliminates the geometry mismatch problem.



3. SMARTDrive 4.0 is also a write-behind cache. It adds significant performance improvement when files are being written. An application writes data to what it thinks is the disk. SMARTDrive takes this data and place it in a cache rather than physically writing it to the disk. This write-behind data stays in the cache until one of the following events occurs:
   
   
   • The cache fills up. The oldest block in the cache is freed up and if it is write-behind data, it is physically written to the disk.
   
   
   
   • The system goes idle. SMARTDrive writes the oldest write-behind data block to the physical disk. As long as the system is idle, it continues to write data until all the write-behind data has been written. This includes both Windows and MS-DOS applications being idle.
     
     When a disk-reset operation occurs (INT21H, function 0DH), SMARTDrive writes all write-behind data to the physical disk. This is a synchronous operation -- SMARTDrive does not give up control until all data has been written. Because of this feature, disk defragmenting programs and other utilities (such as Chkdsk) do not get the benefit of write-behind caching, but this feature provides safety because this is a common way to "commit files" (that is, to make sure that all writes have been completed) under MS-DOS. A number of applications force a system reboot, and they generally issue a disk reset prior to jumping to FFFF:0.
   
   
   
   • If a block is older than 5 seconds, it is written to the physical disk.
   
   
   
   NOTE: SMARTDrive cannot flush its buffers when exiting Windows if you are running a version of QEMM earlier than 6.02. Earlier versions of QEMM trap the INT 2Fh shutdown broadcast. After exiting Windows in this configuration, SMARTDrive does not increase its cache size to the size prior to starting Windows, nor is the cache flushed.



4. SMARTDrive 4.0 implements a shrink algorithm that frees memory for Windows that is similar to the one in SMARTDrive 3.x. The difference is that SMARTDrive 4.0 watches for the Windows startup broadcast while SMARTDrive 3.x provides an IOCTL interface. The net effect is identical but the SMARTDrive 4.0 code is much simpler. When you exit Windows, the process is reversed and the memory is reacquired by SMARTDrive 4.0.

Testing the Performance of SMARTDrive 4.0

1. Hard disk performance tests do not produce representative results because they often use the disk-reset function. These are designed to test the physical performance of the disk, not the disk cache. Disk defragmenting programs and other utilities that directly manipulate the disk (such as CHKDSK) also issue a disk reset to make sure that the data is really written to the disk. These does not show performance gains with write-behind caching.



2. Cache size. SMARTDrive automatically determines a reasonable cache size based on the amount of free extended memory when it initially loads. Any tests should load SMARTDrive with the cache size specified on the command line. A small cache does not perform as well as a large cache.



3. Windows startup speed is not a good test of SMARTDrive's performance because the shrink algorithm causes the cache to be flushed.

Double Buffering and Bus Masters

DEVICE=SMARTDRV.EXE /DOUBLE_BUFFER+

   Copyright 1991,1992 Microsoft Corp.

   Cache size: 1,048,576 bytes
   Cache size while running Windows: 1,048,576 bytes


                Disk Caching Status

   drive   read cache   write cache   buffering
   --------------------------------------------
   A:       yes           no           no
   B:       yes           no           no
   C:       yes           yes          yes
   D:       yes           yes          -

       Microsoft SMARTDrive Disk Cache version 4.00

   For help, type "Smartdrv /?". 

Additional query words: 3.10 3.1 SMARTDRV.EXE smart drive 3.11

Keywords :
Version : WINDOWS:3.1,3.11
Platform : WINDOWS
Issue type :