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This speeds up reads for sequential data, but there is little improvement when accessing random data. • No Read Ahead—Disables the Read-Ahead capability. • Adaptive Read Ahead—Begins using Read-Ahead if the two most recent disk accesses occurred in sequential sectors. If the read requests are random, the controller reverts to No Read Ahead mode. NOTE icon NOTE: The default read cache setting for virtual disks is Adaptive Read Ahead. Reconfiguration Of Virtual Disks An online virtual disk can be reconfigured in ways that expands its capacity and/or change its RAID level.NOTE icon NOTE: Spanned virtual disks such as RAID 10, 50, and 60 cannot be reconfigured. NOTE icon NOTE: Reconfiguring Virtual Disks typically impacts disk performance until the reconfiguration operation is complete. Online Capacity Expansion (OCE) can be done in two ways: • If there is a single virtual disk in a disk group and free space is available, the virtual disk’s capacity can be expanded within that free space. • If a virtual disk is created and it does not use the maximum size of the disk group, free space is available. Free space is also available when a disk group’s physical disks are replaced by larger disks using the Replace Member feature. A virtual disk's capacity can also be expanded by performing an OCE operation to add more physical disks. RAID Level Migration (RLM) refers to changing a virtual disk’s RAID level. Both RLM and OCE can be done at the same time so that a virtual disk can simultaneously have its RAID level changed and its capacity increased. When a RLM/OCE operation is complete, a reboot is not required. See the following table for a list of RLM/OCE possibilities. The source RAID level column indicates the virtual disk RAID level before the RLM/OCE and the target RAID level column indicates the RAID level after the operation has completed. NOTE icon NOTE: If the controller already contains the maximum number of virtual disks, you cannot perform a RAID level migration or capacity expansion on any virtual disk. NOTE icon NOTE: The controller changes the write cache policy of all virtual disks undergoing a RLM/OCE to Write-Through until the RLM/OCE is complete. Table 3. RAID Level Migration Source RAID Level Target RAID Level Number of Physical Disks (Beginning) Number of Physical Disks (End) Capacity Expansion Possible Description RAID 0 RAID 0 1 2 or more Yes Increases capacity by adding disks RAID 0 RAID 1 1 2 No Converts non- redundant virtual disk into a mirrored virtual disk by adding one disk. RAID 0 RAID 5 1 or more 3 or more Yes At least one disk needs to be added for distributed parity data. RAID 0 RAID 6 1 or more 4 or more Yes At least two disks need to be added for dual distributed parity data. RAID 1 RAID 0 2 2 or more Yes Removes redundancy while increasing capacity. RAID 1 RAID 5 2 3 or more Yes Maintains redundancy while doubling capacity. RAID 1 RAID 6 2 4 or more Yes Two disks required to be added for distributed parity data. RAID 5 RAID 0 3 or more 3 or more Yes Converts to a non- redundant virtual disk and reclaims disk space used for distributed parity data. RAID 5 RAID 5 3 or more 4 or more Yes Increases capacity by adding disks RAID 5 RAID 6 3 or more 4 or more Yes At least one disk needs to be added for dual distributed parity data. RAID 6 RAID 0 4 or more 4 or more Yes Converts to a non- redundant virtual disk and reclaims disk space used for distributed parity data. RAID 6 RAID 5 4 or more 4 or more Yes Removes one set of parity data and reclaims disk space used for it. RAID 6 RAID 6 4 or more 5 or more Yes Increases capacity by adding disks NOTE icon NOTE: The total number of physical disks in a disk group cannot exceed 32. You cannot perform RAID level migration and expansion on RAID levels 10, 50, and 60. Fault Tolerance The list of features of the PERC cards that provide fault tolerance to prevent data loss is as follows: • Support for Self Monitoring and Reporting Technology (SMART) • Support for Patrol Read • Redundant path support (for PERC H810 only) • Physical disk failure detection • Physical disk rebuild using hot spares • Controller Cache Preservation • Battery and Non-Volatile Cache backup of controller cache to protect data • Detection of batteries with low charge after boot up The next sections describe some methods to achieve fault tolerance. The SMART Feature The SMART feature monitors certain physical aspects of all motors, heads, and physical disk electronics to help detect predictable physical disk failures. SMART-compliant physical disks have attributes for which data can be monitored to identify changes in values and determine whether the values are within threshold limits. Many mechanical and electrical failures display some degradation in performance before failure. A SMART failure is also referred to as a predicted failure. There are numerous factors that relate to predicted physical disk failures, such as a bearing failure, a broken read/write head, and changes ...

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