SCSI Toolbox :: Newsletter - September 2003
IN THIS EDITION
New - Corporate Licensing options for STB Products
Drive Screening Test
Ask Dr. SCSI - What happens in Audio/Video Optimization?
Did you know? - Repairing DLT Directory
Tape Drives
Corporate Licensing
for STB Products
Could your company use more
than a combined 25 licenses of the SCSItoolbox and/or Developer
Toolbox? Are you tired of using the hardware keys that are required
for the Toolbox Products? You might want to take a closer look
at our Corporate licensing options that include features like:
- Unlimited Corporate usage of SCSI Toolbox Products
- Special OEM build of STB products eliminates hardware keys
- Surprisingly affordable pricing structure
- Onsite Training
Contact Sales to find out if a Corporate Site License for SCSI Toolbox Products is right for you.
The new Drive Screening test is a multi-bus, multi-drive test designed for quickly testing large numbers of disk drives.
What the test does
The Drive Screening test will scan all SCSI and FibreChannel busses in your system, and will identify all disk drives that are connected. Once all disks are discovered the INQUIRY data, drive serial number, capacity, and number of defects is displayed.
When the test is started, the SCSI VERIFY command is sent to all blocks on all drives. The test will fail on two conditions:
- If the VERIFY command fails, the test fails that drive. All other drives remain testing.
- If the number of grown defects increases to a value of 1 or more, the test fails that drive. All other drives remain testing.
At the conclusion of the test, the results may be saved to a file by clicking the Results->File button.
Ask Dr. SCSI - What happens in Audio/Video optimization?
Q. What happens in Audio/Video optimization?
A. Great question, please read the white paper I've written here: A/V
Optimization white paper
Did you know? - Repairing DLT Directory
Did you know 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!
Tape Drives
A tape drive is a data storage device that stores data on magnetic tape.
Magnetic tape is a storage medium made of an oxide layer bonded to a flexible substrate. Data is written or read from the tape medium as it is moved physically across the tape heads.
The tape heads are coils of wire (inductors). When a magnetic field is moved across a coil a small electrical current is generated. Data is stored or represented by various patterns in these magnetic fields.
Key features of tape drives
1.Tape drives are removable-media devices.-
Physical alignment of drive to medium must be maintained
-
Contamination on the medium can be spread to many drives
-
Tapes will be subject to many varying physical environments (heat, humidity, etc)
2.There is a lot of physical handling of the medium in tape drives
- The tape rubs across rollers, heads, etc
- The loading/unloading process can subject the tape to stretching
- Loading/unloading problems can crease or fold the tape
- Contamination can enter drive and/or medium
- Oxide and tape coatings can come off and clog heads, leading to
- Unreadable data
- Scratching the next tape loaded
3. What can go wrong?
- Irregularities in oxide coating can effect base level signals
- Physical change in the medium (stretching, contracting) can change data pattern
- Debris can weaken data signal, or scratch oxide coating
- Physical variations in tape speed can degrade or change data pattern
- Misalignment between head and medium can make data unreadable
- Vast speed differences between data -> drive versus drive -> medium can cause data overflow or underflow problems.
- Data can be lost between cache and tape medium
4. SCSI tape operations are much more complex than disk operations.
- physical restrictions cause programming problems
- end of tape, unexpected end of data
- rewind times, repositioning times, load/unload times
- data blocks can be fixed size or variable
- read and write operations are different for fixed and variable
- Applications software must deal with multiple program access to the drive
SCSI tape operations
-
INQUIRY byte 0 of inquiry data indications device type 1 (sequential access device)
-
TEST UNIT READY indicates when tape is loaded and ready
-
MODE SENSE bytes 5-7 of Block Descriptor indicates block size (if fixed), zero indicates variable block mode
-
MODE SELECT bytes 5-7 of Block Descriptor sets block size
-
REWIND positions tape to physical beginning of tape
-
WRITE writes blocks of data (fixed mode) or bytes of data (variable mode)
-
READ reads blocks of data (fixed mode) or bytes of data (variable mode)
-
SPACE positions tape forward or reverse by block, filemark, sequential filemark, setmark
-
UNLOAD unloads tape from drive
-
LOG SENSE retrieves statistical operational data about drive and medium
-
WRITE FILEMARK writes filemark(s) to tape, flushes cached write data
-
ERASE (takes A LONG TIME! And isn’t interruptible!)
- PREVENT/ALLOW MEDIUM REMOVAL
-
LOCATE
Some common read error conditions –
| Read Type | Error Condition | Sense Key | Information Bytes | Tape Position |
| V w/o SILI | Requested length Not equal to actual Length |
No Sense, ILI | Residue in bytes | After Block |
| Any | Filemark encountered | No Sense, FM | Residue in blocks | After FM |
| Any | EOD encountered | Blank Check | Residue in blocks | At EOD |
| Any | EOT encountered | Medium Error, EOM | Residue in blocks | Undefined |
| F | Block is variable | Illegal Request | None | Unchanged |
| F | SILI bit set in CDB | Illegal Request | None | Unchanged |
