In the RAID environment, data is striped across multiple physical hard drives. The array is defined as the set of hard drives included in the data striping.
Data Scrubbing forces all data sectors in a logical drive to be accessed so that sector media errors are identified and corrected at the disk level using disk ECC information if possible, or at the array level using RAID parity information if necessary. For a high level of data protection, Data Scrubbing should be performed weekly.
The array spcifies which drives should be included in the striping of data. Each array is subdivided into one or more logical drives. The logical drives specify the following:
RAID level 0 stripes the data across all of the drives of the array. RAID-0 offers substantial speed enhancement, but provides for no data redundancy. Therefore, a defective hard disk within the array results in loss of data in the logical drive assigned level 0, but only in that logical drive.
RAID level 1 provides an enhanced feature for disk mirroring that stripes data as well as copies of the data across all the drives of the array. The first stripe is the data stripe, and the second stripe is the mirror (copy) of the first data stripe The data in the mirror stripe is written on another drive. Because data is mirrored, the capacity of the logical drive when assigned level I is 50% of the physical capacity of the grouping of hard disk drives in the array.
RAID level 5 stripes data and parity across all drives of the array. When a disk array is assigned RAID-5, the capacity of the logical drive is reduced by one physical drive size because of parity storage. The parity is spread across all drives in the array. If one drive fails, the data can be rebuilt. If more than one drive fails, but one or none of the drives are actually defective, then data may not be lost. You can use a process called software replacement on the non-defective hard drives.
A Software Replace of a hardfile refers to when the hardfile is not physically replaced in the system. A drive may have been marked defunct but brought back online using the RAID Administration program. The drive is rebuilt without having been physically replaced. This could occur because when the RAID Adapter communicates with the hardfile and receives an unexpected response, the adapter will mark the drive defunct in order to avoid any potential data loss.
Synchronization reads all the data bits of the entire logical drive, calculates the parity bit for the data, compares the calculated parity with the existing parity, and updates the existing parity if inconsistent.
The following definitions describe the logical drive states for the IBM SCSI-2 F/W PCI-Bus RAID Adapter and the IBM F/W Streaming RAID Adapter/A:
The adapters also assign device states to physical drives. The following definitions describe these device states:
The RAID adapter marks an ONL or OFL (RBL if ServeRAID) drive defunct, changing its status to DDD and removing power from the drive, when one of the following conditions occur:
A hot-spare or standby hot-spare drive (see below) enters the defunct hot-spare (DLIS) state if it fails to respond to the adapter commands. Once a DHS drive is replaced, its state changes from DHS to HSP. Only the IBM ServeRAID Adapter has the DHS state.
No device is present in the bay or the adapter cannot communicate with the drive. This state is represented with dashes (- - -) on the IBM ServeRAID configuration screen, or a blank space on the Administration and Monitor screen. Only the JBM ServeRAID Adapter has this state.
Format: the drive is being reformatted.
HSP (IBM ServeRAID & ServeRAID II)
A hot-spare (HSP) drive is a drive designated to be a replacement for the first DDD drive that occurs. The state of the drive appears as HSP. When a DDD drive occurs and a HSP is defined, the hot-spare drive takes over for the drive that appears as DDD. The HSP drive is rebuilt to be identical to the DDD drive. During the rebuilding of the HSP drive, this drive changes to the RBL state. The RBL state will turn to ONL once the drive is completely rebuilt and fully operating for the DDD drive.
HSP (Other controllers)
A hot-spare (HSP) drive is a drive designated to be a replacement for the first DDD drive that occurs. The state of the drive appears as HSP. When a DDD drive occurs and a HSP is defined, the hot-spare drive takes over for the drive that appears as DDD. The HSP drive is rebuilt to be identical to the DDD drive. During the rebuilding of the HSP drive, this drive changes to the OFL state. The OFL state will turn to ONL once the drive is completely rebuilt and fully operating for the DDD drive.
Offline, a good drive that replaces a defunct drive in a RAID level I or level 5 array. This drive is associated with the array, but does not contain any data. Drive status remains OFL during the rebuild phase.
Online, a drive that RAID adapter detects as installed, operational, and configured into an array appear as this state.
The firmware of a hard drive uses algorithms to track the error rates on the drive. The drive alerts the user with a Predictive Failure Analysis (PFA) alert via the RAID administration utility and NetFinity when degradation of drive performance (read/write errors) is detected. When a PFA alert occurs, physical replacement of the drive is recommended.
A drive in this state is being rebuilt. Only the IBM ServeRAID Adapter has this state. A physical hard drive can enter the RBL state if one of the following conditions occurs: A good working drive replaces a DDD drive that is part of the critical logical drive. At the end of a successful rebuild, the state of the physical drive changes to ONL, and the state of the corresponding logical drives changes to OKY.
The HSP or standby hot-spare (SHS) drive is added to the array and the state changes from HSP or SHS to RBL. At the same time, the DDD drive is removed from the array and its state changes to DHS from DDD. The adapter then automatically reconstructs data in the RBL drive. The state of the corresponding logical drive remains CRT (if the RAID level is 1 or 5) or OFL during the rebuild process. When the rebuild completes successfully, the device state changes from RBL to ONL and the logical drive state changes from CRT to OKY.
A ready (RDY) or standby (SBY) drive replaces a DDD drive that is part of the critical logical drive. The state of the RDY or SBY drive becomes RBL. When the rebuild completes successfully, the state changes to ONL. The DDD drive is removed from the logical drive and becomes DHS
RDY appears as the status of a drive that the RAID adapter detects as installed, spun up, but not configured in an array.
A standby drive is a hard disk drive that the RAID adapter has spun down. Devices such as tape drives and CD-ROM drives are also considered to be in a standby state. Only the IBM ServeRAID Adapter has the state.
A standby hot-spare is a hot-spare drive that the adapter has spun down. If a drive becomes defrinet and no suitable hot-spare drive is available, a standby hot-spare of the appropriate size spins up and enters the RBL state. You must have at least four hard disk drives if you want a standby hot-spare with RAID-5. Only the IBM ServeRAID Adapter has the state.
Unformatted, a drive that requires a low-level formatting before it can be used in an array. You can start the low-level format by selecting Format Drive from the RAID Configuration Main Menu.
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