SMARTDrive 4.0 Design Overview

• Microsoft Windows operating system 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

Certain disk controllers support a concept called bus mastering. This is where the actual disk controller takes over the bus in order to transfer data to or from system RAM. Some SCSI controllers have this feature. A problem occurs when running in the virtual 8086 mode that Windows 3.0 and 3.1 virtual machines provide. Memory managers such as QEMM and EMM386.EXE also use virtual 8086 mode. The read or write address that is passed to MS-DOS is often not the same as the actual physical memory address. This can cause data to be read from the wrong location or cause data to be written to the wrong address, which in turn can cause erratic system behavior, general protection faults, and the system to stop responding (hang).

Microsoft created a standard called Virtual DMA Services, which provides an interface that allows these bus-master controllers to get the correct address and avoid the problems mentioned above. However, some older bus- master controller cards do not support this standard. To allow SMARTDrive to work with these older bus mastering cards, a feature has been added to SMARTDrive that provides a memory buffer that has the same physical and virtual addresses. This avoids the system instability problem at the cost of 2.5K of conventional memory and a small amount of performance (the cost of moving the data to and from the buffer.) To use this feature, place the following line in the CONFIG.SYS file:

   DEVICE=SMARTDRV.EXE /DOUBLE_BUFFER+

Most disk controllers do not need double buffering. This includes all MFM, RLL, and IDE controllers as well as many ESDI and SCSI devices. The Windows 3.1 Setup program will not install the double buffer driver in most cases. In the cases where Setup is unable to determine if double buffering is needed or not, it will install the driver based on the reasoning that it is better to error on the side of safety.

A feature has been added to SMARTDrive to help determine if double buffering is unneeded and allow removal of the driver. Once the system is running with SMARTDrive loaded, type "SMARTDRV" (without quotation marks) at the MS-DOS command prompt. The following will appear:

   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 /?".

To determine if double buffering is required, look at the column labeled buffering. For each drive that is being cached, it can have one of three values: yes, no, or -. "Yes" indicates that double buffering is needed and being performed; "no" indicates that buffering is not needed. "-" indicates that SMARTDrive has not yet determined the necessity of double buffering.If the buffering column has all "no" entries in it, the double buffer driver is unneeded and can be safely removed from the CONFIG.SYS file.