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MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID
| Components and Features
2.4.7 Patrol Read 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 Section5.8, Controller Property-Related Options or Section9.5,
Running a Patrol Read.
2.4.8 Disk Striping 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.
Figure 3: Example of Disk Striping (RAID 0)
2.4.8.1 Stripe Width
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.
2.4.8.2 Stripe Size 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.
2.4.8.3 Strip Size The strip size is the portion of a stripe that resides on a single drive.
2.4.9 Disk Mirroring 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.
Segment 1
Segment 5
Segment 9
Segment 2
Segment 6
Segment 10
Segment 3
Segment 7
Segment 11
Segment 4
Segment 8
Segment 12
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