Dell PowerVault TL2000 - Manual (Page 7)

Performance Considerations for Tape Drives and Libraries
7
General Hard Drive Configuration Considerations
Data/operating system (OS) on different LUNs. Backing up data on a logical unit number (LUN)
separate from the OS LUN ensures that the hard drive is not splitting access and overhead between
OS operations and backup operations. This can be accomplished by having one hard drive or disk
array contain the OS and a physically separate hard drive or disk array contain the data
to be backed up.
Figure 1-1. Single-Channel vs. Two-Channel Bandwidth
Hard Drive Performance
By design, tape drives write data sequentially and require a constant data feed to keep the drive
operating sequentially (avoiding back hitching). Conversely, hard drives are random access devices.
Therefore, hard drives can sometimes struggle to provide sequential data to tape drives if that data
is spread out over the drive platter. This forces the drive to continuously seek small blocks of data.
Additionally, other hard drive attributes can further affect the throughput of data to the tape drive.
Spindle speed. Typically measured in RPMs (revolutions per minute), the hard drive's spindle
speed determines how many times per minute the drive platter assembly can perform a full
revolution. This has a direct effect on both random access times and sequential transfer rates.
The higher the spindle speed, the faster the drive can access data.
Single Shared LUN Separate LUNs
Single LUN with
Backup data
and OS
OS LUN
SCSI or RAID
Controller
Tape Drive
Backup Data
LUN
SCSI or RAID
Controller
Tape Drive