Western Digital WDE2170 - Manual (Page 30)

Advanced Product Features WD Enterprise WDE2170/WDE4360
RELEASED 04-08-97 4079-001045 Rev. B 22
3.8. Command Queuing and Reordering
The drives support both tagged and untagged queuing. Tagged queuing allows the drive
to receive multiple I/O processes from each initiator. Untagged queuing allows the drive
to receive a maximum of one I/O process from each initiator.
In order to use tagged command queuing, the initiator must set the disconnect privilege
bit. This allows the drive to disconnect from the SCSI bus. It also allows the same or
other initiators to connect to the drive and send additional commands. The drive returns
a status of BUSY if the disconnect privilege bit is off and the drive has I/O processes in
the command queue. The drive has the capability to queue up to 64 I/O processes. Upon
receipt of the 65th I/O process, the drive returns a status of QUEUE FULL.
The drive can reorder I/O processes in the command queue. The order in which the
drive executes I/O processes may differ from the order in which they were received.
Command reordering employs an algorithm which improves the throughput of the drive
by attempting to minimize both the seek time and the rotational latency.
The drive uses a command aging feature to prevent the command reordering algorithm
from keeping I/O processes waiting in the command queue for extended periods of time.
The drive allows I/O processes to wait in the command queue for only a specified length
of time before they are scheduled in the next I/O process.
3.9. Media Defect Management
Western Digital drives support a defect management algorithm that uses sector slipping.
Defective sites are pushed down during a reassignment operation to maintain a
sequential order. Spare sectors are placed throughout the drive to support this algorithm.
This routine can be invoked by the drive when Auto-Read Reallocation/Auto-Write
Reallocation (ARRE/AWRE) is enabled in MODE SELECT Page 1. If these functions
are disabled, the drive reports status and may recommend that the host request a
reassign.
3.10. Microcode Download
Western Digital drives support downloadable microcode via the SCSI WRITE BUFFER
command. The supported modes include Download Microcode and Save, as well as
Download Microcode with Offsets and Save. Download Microcode with Offsets is used
when the buffer is limited to the full download and requires smaller partial downloads to
complete the download task.
3.11. Reed Solomon ECC On-the-Fly
Error Correction Code (ECC) on-the-fly is a correction technique that reduces the
uncorrectable read error rate in hardware. This provides a high degree of data integrity
with no impact on the drive’s performance.
3.12. CRC Data Protection
Cyclic Redundancy Check (CRC) data protection is a feature that enhances data
integrity. During a write operation, before a user data block enters the cache (data
buffer), the SCSI controller generates CRC data (2 bytes) and attaches it to the user data
block (usually 512 bytes). During a read operation, the SCSI controller reads each user
data block and its CRC data in the cache, then verifies the CRC data as it transfers each
user data block to the SCSI bus.