Dell PowerVault 221S (SCSI) - Manual (Page 33)

A REFERENCE GUIDE FOR OPTIMIZING DELL™ SCSI SOLUTIONS VER A02
PAGE 33 11/17/2005
For more details on rebuild process please refer to Appendix A –
Reference 5: Maintenance Best Practices for Direct Attached SCSI
Solutions
• Patrol Read
o Patrol Read is a preventative maintenance background operation
(available only on PERC 3 (except PERC 3/DI), PERC 4 and PERC 4e
controller families running 3.0 and higher firmware).
o Patrol Read examines each block in a configured physical disk for media
errors and attempts to fix media errors by reallocating the bad block.
o Patrol Read is designed to run so as to not impact I/O performance.
o Patrol Read has two run modes: Manual and Automatic. The mode is set
from within the controller BIOS. By default the run mode is set for
“Automatic”.
o Highly recommended as part of regularly scheduled system
maintenance to prevent downtime due to media errors.
NOTE: Patrol Read and Consistency Check are not the same operations;
Patrol Read examines the physical disk for media defects while Consistency
Check validates data using data parity, to help prevent data errors.
High Availability Needs – Clustering
• Clustering is supported only on some HBAs – see Table 5-2. Clustering is not
supported on embedded or non-RAID controllers.
• Multiple logical volumes on the same physical array are not supported in
clustering environment – only one logical volume/array.
Heterogeneous vs. Homogeneous environments
• SCSI Technology (U320/U160/Ultra 2)
It is possible to mix a combination of hard drives compliant with U320, U160
or Ultra-2 generation of SCSI technologies. Mixing of hard drives is supported
as long as all the devices on the SCSI bus have Low Voltage Differential
(LVD) signaling.
Though possible, it is not recommended to mix drives, since overall SCSI bus
performance will be limited by the slowest technology present in the mix of
array. Where mixing of HDD technology is not avoidable, it is a best practice
to have all hard drives in any particular array be of matching technology. In all
cases, the overall subsystem performance will be limited to some degree by
the slowest technology present in the mix.
• Spindle Speed
It is recommended that all hard drives within any particular array be of the
same spindle speed (RPM – which can vary from 10,000-15,000 on the hard