SCSI Toolbox :: Newsletter - July 2003

IN THIS EDITION
4.1 Release of the Toolbox (7/15/03)
Threaded Commands in DTB!
Ask Dr. SCSI - What's the deal with transfer rates anyway?
Did you know? - Repair DLT directory




4.1 STB Release
This will be in the STB v4.1 release which will be released on July 15.

1. New "Tape Speed Test"
2. New LTO tape cartridge display
3. New Disk Manufacturing/Screening module - can format within test sequence
4. New Disk Manufacturing/Screening module - can change blocksize within
test sequence
5. SWP & DTB - Fixed buffer mode bug in SetTapeBlocksize()
6. SWP & DTB New - added GetBufferMode() function
7. SWP & DTB New - added SetBufferMode() function
8. SWP & DTB New - added MultiTasking CDB's
9. DTB New - Visual C++ DTB library released
10. Tape Performance Test - accuracy enhanced
11. Fixed - SATA & ATA RAID crash issue
12. Users Application Guide

1. Issue the command - the function returns immediately with a Thread ID (TID)
2. To see what the command is doing you use the GetTIDStatus function. This will tell you if the command is still pending, or if the command is complete it will tell if it completed successfully or if it got an error. Detailed error information as well as command completion time is available for every TID.
3. Once a TID is complete call the FreeTID function to release the system resources.

Q. What's the deal with transfer rates anyway?

A. Good question! I like to think about transfer rates in two ways, "Real world" and "Theoretical".

Real world transfer rates are what you are going to see in, you guessed it, the real world! For instance, if it takes one minute to write 720MB of data to your disk drive, your "real world" transfer rate is 12MB/sec. Or, if it takes 20 minutes to backup 3GB to your DVD drive, you are seeing a "real world" transfer rate of about 2.5 MB/sec. Real world numbers reflect the average amount of data transferred over a given amount of time, and therefore take into account all drive and system activities that occur between actual data transfers.

Theoretical transfer rates are almost always referring to the instantaneous burst transfer rate when the device is in DATA IN or OUT phase. These rates are almost always much higher than the real world rates. Why? Because they are only measuring the transfer rate when data is being transferred - they don't take into effect how often these transfers can take place. With disk drives, head movement, occasional recalibration, seeks needed to access spare blocks, rotational latency, and cache misses can all contribute to put a significant amount of time between data transfers. On tapes there can be repositioning due to data underflow, retries and mapping of errors, rewinds - all sorts of delays between actual data transfers.

"So" you ask, "where can I see these different transfer rates with the SCSItoolbox?"
Basically, the numbers reported with DriveWatch are going to be burst rate (theoretical) numbers - these are going to show the time the actual read or write commands took to transfer their data. The numbers shown in the Disk Read or Write Performance tests, or the are going to be average-over-time (real world) rates.











Did you know? - Repair DLT directory
That DLT tapes keep a directory of the tape contents at the beginning of the tape. This helps the drive perform seek and positioning commands more efficiently. This directory is updated any time the tape is unloaded from the drive. But what happens if you are writing data to the tape, and lose power before unloading the tape? The tape directory gets corrupted is what happens! But not to worry, just use the Repair DLT directory function to fix it!

This function will tell you if the directory is corrupted, and if it is it will fix it. Depending on how much data is on the tape, this function can take a long time! But if your tape directory is corrupted, it’s worth the wait!