Description
Common definitions and drive states used in relation to MegaRAID RAID Controllers.
Product
- MegaRAID Storage Manager
- LSI Storage Authority
Virtual Drive
A virtual drive is a partition in a drive group that is made up of contiguous data segments on the drives. A virtual drive can consist of these components:
- An entire drive group
- A part of a drive group
- A combination of any two of these conditions
Drive Group
A drive group is a group of physical drives. These drives are managed in partitions known as virtual drives. You can create one or more virtual drives on a group of drives attached to a controller card. However, this is based on the support of sliced VD and RAID level of the controller.
Virtual Drive States
A virtual drive property indicating the condition of the virtual drive. A virtual drive can be in any one of the following states
Optimal
A virtual drive whose members are all online. In the output of the StorCLI commands, Optimal is displayed as optl.
Degraded
A virtual drive with a redundant RAID level with one or more member failures and can no longer sustain a subsequent drive failure. In the output of the StorCLI commands, Degraded is displayed as dgrd.
Partially Degraded
A virtual drive with a redundant RAID level that is capable of sustaining more than one member drive failure. This state also applies to the virtual drive’s member drives.Currently, a RAID 6 or RAID 60 virtual drive is the only virtual drive that can be partially degraded.In the output of the StorCLI commands, Partially Degraded is displayed as Pdgd.
Failed
The virtual drive has failed.
Foreign
A foreign configuration is a RAID configuration that already exists on a replacement set of drives that you install in a computer system. WebBIOS Configuration Utility and the MegaRAID Storage Manager software allows you to import the existing configuration to the RAID controller or clear the configuration so you can create a new one.
Offline
The virtual drive is not available to the RAID controller, one or more member failures making the data inaccessible. In the output of the StorCLI commands, Offline is displayed as OfLn.
Physical Drive States
A physical drive can be in any one of the following states:
Unconfigured Good
A drive accessible to the RAID controller but not configured as a part of a virtual drive or as a hot spare.In the output of the StorCLI commands, Unconfigured Good is displayed as UGood.
Hot Spare
A drive that is configured as a hot spare.
Online
A drive that can be accessed by the RAID controller and will be part of the virtual drive.In the output of the StorCLI commands, Online is displayed as onln.
Rebuild
A drive to which data is being written to restore full redundancy for a virtual drive.
Failed
A drive that was originally configured as Online or Hot Spare, but on which the firmware detects an unrecoverable error.
Unconfigured Bad
A drive on which the firmware detects an unrecoverable error; the drive was Unconfigured Good or the drive could not be initialized.In the output of the StorCLI commands, Unconfigured Bad is displayed as UBad.
Note: If an improperly removed device is reconnected to the RAID controller it will be recognized as UBAD (Unconfigured Bad). It can be incorprated back into the the Virtual Drive by setting to UGOOD
Missing
A drive that was Online, but which has been removed from its location.In the output of the StorCLI commands, Missing is displayed as Msng.
Offline
A drive that is part of a virtual drive but which has invalid data as far as the RAID configuration is concerned. In the output of the StorCLI commands, Offline is displayed as offln.
None
A drive with an unsupported flag set. An Unconfigured Good or Offline drive that has completed the prepare for removal operation.
Definitions
RAID
Redundant Array of Independent Disks is a group of multiple, independent drives that provide high performance by increasing the number of drives used for saving and accessing data. A RAID drive group improves input/output (I/O) performance and data availability. The group of drives appears to the host system as a single storage unit or as multiple virtual drives. Data throughput improves because several drives can be accessed simultaneously. RAID configurations also improve data storage availability and fault tolerance. Redundant RAID levels (RAID levels 1, 5, 6, 10, 50, and 60) provide data protection.
Selecting a RAID Level
To make sure of the best performance, you must choose the optimal RAID level when you create a system drive. The optimal RAID level for your drive group depends on a number of factors:
- The number of drives in the drive group
- The capacity of the drives in the drive group
- The need for data redundancy
- The disk performance required
RAID 5
Uses data striping and parity data across three or more drives (distributed parity) to provide high data throughput and data redundancy, especially for applications that require random access. RAID 5 can survive the failure of one drive. If 2 or more drives fail then a total delete and rebuild is required.
RAID 6
Uses data striping and parity data across three or more drives (distributed parity) to provide high data throughput and data redundancy, especially for applications that require random access. RAID 6 can survive the failure of two drives. If 3 or more drives fail then a total delete and rebuild is required.
Maximizing Fault Tolerance
Fault tolerance is achieved through the ability to perform automatic and transparent rebuilds using hot spare drives and hot swaps. A hot spare drive is an unused online available drive that the RAID controller instantly plugs into thesystem when an active drive fails. After the hot spare is automatically moved into the RAID drive group, the failed drive is automatically rebuilt on the spare drive. The RAID drive group continues to handle requests while the Rebuildoperation occurs.A hot swap is the manual substitution of a replacement unit in a disk subsystem for a defective one, where the substitution can be performed while the subsystem is running hot swap drives. An Auto-Rebuild feature in the WebBIOS Configuration Utility allows a failed drive to be replaced and automatically rebuilt by “hot-swapping” the drive in the same drive bay. The RAID drive group continues to handle requests while the Rebuild operation occurs, which provides a high degree of fault tolerance and zero downtime.
Hot spare
A standby drive that can automatically replace a failed drive in a virtual drive and prevent data from being lost. A hot spare can be dedicated to a single redundant drive group or it can be part of the global hot spare pool for all drive groups controlled by the controller. When a drive fails, MegaRAID Storage Manager or LSI Storage Authority software automatically uses a hot spare to replace it and then rebuilds the data from the failed drive to the hot spare. Hot spares can be used in RAID 1, 5, 6, 10, 50, and 60 storage configurations.A hot swap manually replaces a defective drive unit when the computer is still running. When a new drive is installed, a rebuild occurs automatically if these situations occur:
- The newly inserted drive is the same capacity as or larger than the failed drive.
- The newly inserted drive is placed in the same drive bay as the failed drive it is replacing.
You can configure the controller to detect the new drives and automatically rebuild the contents of the drive.
Consistency Check
An operation that verifies that all stripes in a virtual drive with a redundant RAID level are consistent and that automatically fixes any errors. For RAID 1 drive groups, this operation verifies correct mirrored data for each stripe
Fast Initialization
The firmware quickly writes zeros to the first and last 8-MB regions of the new virtual drive, and then completes the initialization in the background or with next scheduled Consistency Check. This allows you to start writing data to the virtual drive immediately.
Fault Tolerance
The capability of the drive subsystem to undergo a single drive failure per drive group without compromising data integrity and processing capability. SAS RAID controllers provide fault tolerance through redundant drive groups in RAID levels 1, 5, 6, 10, 50, and 60. They also support hot spare drives and the auto-rebuild feature.
Foreign Configuration
A RAID configuration that already exists on a replacement set of drives that you install in a computer system. MegaRAID Storage Manager software lets you import the existing configuration to the RAID controller, or you can clear the configuration so you can create a new one.
Initialization
The process of writing zeros to the data fields of a virtual drive and, in fault-tolerant RAID levels, generating the corresponding parity to put the virtual drive in a Ready state. Initialization erases all previous data on the drives. Drive groups will work without initializing, but they can fail a consistency check because the parity fields have not been generated
IO policy
A virtual drive property indicating whether Cached I/O or Direct I/O is being used. In Cached I/O mode, all read operations are buffered in cache memory. In Direct I/O mode, read operations are not buffered in cache memory. Data is transferred to cache and the host concurrently. If the same data block is read again, it comes from cache memory. (The IO Policy applies to read operations on a specific virtual drive. It does not affect the read-ahead cache.)
Learning Cycle
An energy pack calibration operation performed by a RAID controller periodically to determine the condition of the energy pack. You can start energy pack learn cycles manually or automatically
Patrol Read
A process that checks the drives in a storage configuration for drive errors that could lead to drive failure and lost data. The Patrol Read operation can find and sometimes fix any potential problem with drives before host access. This enhances overall system performance because error recovery during a normal I/O operation might not be necessary.
Read Policy
A controller attribute indicating the current Read Policy mode. In Always Read Ahead mode, the controller reads sequentially ahead of the requested data and stores the additional data in cache memory, anticipating that the data will be needed soon. This speeds up read operations for sequential data, but you will see little improvement when accessing random data. In No Read Ahead mode (known as Normal mode in WebBIOS), read ahead capability is disabled.
Rebuild
The regeneration of all data to a replacement drive in a redundant virtual drive after a drive failure. A drive rebuild normally occurs without interrupting normal operations on the affected virtual drive, though some degradation of performance of the drive subsystem can occur.
Strip Size
The portion of a stripe that resides on a single drive in the drive group.
Temperature
Temperature of the energy pack, measured in Celsius
Write-Back
In Write-Back Caching mode, the controller sends a data transfer completion signal to the host when the controller cache has received all of the data in a drive write transaction. Data is written to the drive subsystem in accordance with policies set up by the controller. These policies include the amount of dirty/clean cache lines, the number of cache lines available, and elapsed time from the last cache flush.
