Lenovo ThinkServer RD330 MegaRAID SAS Software User Guide - Page 25

MegaRAID SAS Software User Guide 2.4.7 Patrol Read 2.4.8 Disk Striping Chapter 2: Introduction to RAID | Components and Features Patrol read involves the review of your system for possible drive errors that could lead to drive failure and then action to correct errors. The goal is to protect data integrity by detecting drive failure before the failure can damage data. The corrective actions depend on the drive group configuration and the type of errors. Patrol read starts only when the controller is idle for a defined period of time and no other background tasks are active, though it can continue to run during heavy I/O processes. You can use the MegaRAID Command Tool or the MegaRAID Storage Manager to select the patrol read options, which you can use to set automatic or manual operation, or disable patrol read. See Section 5.8, Controller Property-Related Options or Section 9.5, Running a Patrol Read. Disk striping allows you to write data across multiple drives instead of just one drive. Disk striping involves partitioning each drive storage space into stripes that can vary in size from 8KB to 1024 KB. These stripes are interleaved in a repeated sequential manner. The combined storage space is composed of stripes from each drive. It is recommended that you keep stripe sizes the same across RAID drive groups. For example, in a four-disk system using only disk striping (used in RAID level 0), segment 1 is written to disk 1, segment 2 is written to disk 2, and so on. Disk striping enhances performance because multiple drives are accessed simultaneously, but disk striping does not provide data redundancy. 2.4.8.1 Stripe Width 2.4.8.2 Stripe Size 2.4.8.3 Strip Size 2.4.9 Disk Mirroring Figure 3: Segment 1 Segment 5 Segment 9 Segment 2 Segment 6 Segment 10 Example of Disk Striping (RAID 0) Segment 3 Segment 7 Segment 11 Segment 4 Segment 8 Segment 12 Stripe width is the number of drives involved in a drive group where striping is implemented. For example, a four-disk drive group with disk striping has a stripe width of four. The stripe size is the length of the interleaved data segments that the RAID controller writes across multiple drives, not including parity drives. For example, consider a stripe that contains 64 KB of disk space and has 16 KB of data residing on each disk in the stripe. In this case, the stripe size is 64 KB and the strip size is 16 KB. The strip size is the portion of a stripe that resides on a single drive. With mirroring (used in RAID 1 and RAID 10), data written to one drive is simultaneously written to another drive. The primary advantage of disk mirroring is that it provides 100 percent data redundancy. Because the contents of the disk are completely written to a second disk, data is not lost if one disk fails. In addition, both drives contain the same data at all times, so either disk can act as the operational disk. If one disk fails, the contents of the other disk can be used to run the system and reconstruct the failed disk. Page 25