Dell PowerVault 221S Optimizing Dell SCSI Solutions - Page 31

A REFERENCE GUIDE FOR OPTIMIZING DELL™ SCSI SOLUTIONS VER A02 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 PAGE 31 11/17/2005