Microsoft KB Archive/254574

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Description of Advanced SCSI Disk Properties Features

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Q254574

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The information in this article applies to:


 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows 2000 Datacenter Server
 * Microsoft Windows 2000 Professional
 * Microsoft Windows 2000 Server

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SUMMARY
This article describes some of the advanced performance options available if you view the properties of a Small Computer Systems Interface (SCSI) hard disk. These advanced options are only available on SCSI hard disks, not on drives based on Integrated Drive Electronics (IDE) or ATAPI. These options are found in 2 different locations, the Disk Properties tab, and the SCSI Properties tab. This article also explains how these advanced options may effect the overall performance and stability of your computer.

MORE INFORMATION
To view these advanced performance options, double-click My Computer, right-click the SCSI hard disk you want to view, click Properties, click the Hardware tab, and then click Properties.

The Disk Properties Tab "Write Cache Enabled" Feature
If you enable this feature, your computer sends an enable-write-cache command to the hard disk activating the hard disk write-back cache, and if you disable this feature, the hard disk write-back cache is deactivated. When you enable this feature, you receive a warning message that says enabling this option could lead to file system damage or data loss. This could happen if a computer or power failure occurs that would prevent your computer from shutting down properly. This is because until the cached data is written to disk, the last few write operations were reported to the operating system as being complete by the hardware, but the hardware still contains the data in its physical memory. Should a power loss occur, those last few write operations may not take place, possibly leading to data loss or file system damage. However, if the data contained on the disk is temporary data (such as a paging file) that can be easily reproduced, or read-only data that can be restored in the event of data damage due to the write cache being enabled, then you may see an increase in disk Input/Output (I/O) performance by enabling it. In this case, most write operations would more than likely be to the file system itself to keep track of files last access dates and time stamps, file size changes, and file ownership and security information if on an NTFS partition. Note that this hardware caching option is independent of the operating system NTFS file cache that could also be in use if the drive contains an NTFS partition.

The SCSI Properties Tab "Disable Tagged Queuing" Feature
Tagged queuing is done by the SCSI controller and driver to increase the speed of completing I/O tasks. This is accomplished by sorting the commands so the order of the commands issued to the physical disk drive to read or write results in the most efficient movement of the physical read and write heads. Since moving the physical heads takes time, by intelligently moving them in a sequential order rather than in random order, seek times are minimized, resulting in greater I/O performance.

The SCSI Properties Tab "Disable Synchronous Transfers" Feature
There are two methods of transferring data over SCSI cables, Synchronous mode and Asynchronous mode. Asynchronous mode transfers use an interlocked handshake method where a device (adapter or drive) cannot do the next data transfer operation until it receives positive acknowledgment that the other device received the last data transfer properly. Synchronous transfer mode permits the sending device to send multiple data packets without receiving acknowledgment that the receiver actually received every data packet sent. By doing to, more data can be sent or received before an acknowledgement is required. Disabling either Tagged Queuing or Synchronous transfers results in performance degradation. However, if you suspect a device is having trouble with overlapped requests (queuing) or getting data reliably and quickly (synchronously), you can attempt to use a slower but more reliable mode of operation.

NOTE: Some hard disks do not support the option to disable the write cache on a permanent basis, and report that the write cache is enabled again after the computer is restarted. This is because some drives do not have Non-Volatile Random Access Memory (NVRAM) to enable you to save the disable cache setting.

Other disks may show the Write Cache Enabled check box as not available for disk. This is disabled if the property page cannot retrieve the current disk write cache setting from the disk.

Please be aware of the following issue:

"Q281672 Possible Data Loss After You Enable Write Caching" Additional query words:

Keywords : kbtool KBW2KHOME

Issue type : kbinfo

Technology : kbwin2000AdvServ kbwin2000AdvServSearch kbwin2000DataServ kbwin2000DataServSearch kbwin2000Serv kbwin2000ServSearch kbwin2000Search kbwin2000ProSearch kbwin2000Pro kbWinAdvServSearch kbWinDataServSearch