Dell PowerVault 221S (SCSI) - Manual (Page 31)

A REFERENCE GUIDE FOR OPTIMIZING DELL™ SCSI SOLUTIONS VER A02
PAGE 31 11/17/2005
In contrast, Adaptec controllers will only use space on all ‘child’ arrays
equal to the available space on the smallest ‘child’ array, and will leave
the remaining space on the larger drives unused and available, and the
performance behavior will not be affected as the array fills up.
• Array Reconfiguration / Disk Expansion / Rebuild
o As stated in the ‘Getting Started with RAID’ guide (see Appendix A –
References), the recommended method of changing an array
configuration is to back up the data on the array, create a new array with
the new desired parameters, and then migrate the data to the new array.
In cases where this is not possible, reconfiguring arrays is supported for
the controllers. However, should an array need to be reconfigured, the
reconfiguration paths supported differ on different controllers. Please
refer to the documentation on each controller for information on
supported reconfiguration paths.
o Many factors may influence the time it takes to perform a rebuild or
reconfiguration operation on an array including: Size of disk and array,
RAID level, IO profile, system load, PERC family,
For more details on the rebuild process please refer to Appendix A –
Reference 5: Maintenance Best Practices for Direct Attached SCSI
Solutions
• Controller upgrade / Array Migration / Drive Roaming / metadata
o PERC controllers store array configuration metadata both in controller
NVRAM and on-disk. Arrays are upgradeable and can be migrated within
the family of controllers made by the same vendor; i.e., migration of
arrays from one LSI controller to another or from one Adaptec controller
to another is possible whereas migration between the two families is not;
e.g., an array from an LSI controller cannot be migrated to an Adaptec
controller, or vice-versa.
o Drive roaming is initiated when the HDDs are changed to different SCSI
IDs or channels on a same controller. When the HDDs are placed on
different SCSI IDs or different channels of the same controller, controller
detects the RAID configuration from configuration metadata residing on
the HDD. Drive roaming is supported on all families of SCSI RAID
controllers.
NOTE: In a clustering environment, drive roaming is supported within the
same channel only.
o For additional information on Array Migration or Drive Roaming, please
refer to the User Guide for the controller.
• Write cache policy considerations
o Write back
In write-back caching, data transfer is completed when the controller
cache receives all data from host for the write transaction. Write-back
caching is faster, but on PERC controllers that do not have a battery