This document explains data scrubbing and synchronization are two techniques by which ServeRAID technology can prevent loss of data.
The ServeRAID subsystem is designed to tolerate and correct problems when a single physical disk drive cannot complete a read or write to a stripe unit. A single stripe unit failure is corrected on the fly anytime the error is detected. Data Scrubbing is an adaptive process that runs as a background operation on the ServeRAID controller. It is adaptive, so that during periods of high disk access by the host applications, data scrubbing activity is lessened, and thus does not interfere with the applications.
During periods of lesser disk accesses by host applications, the data scrubbing activity can be increased. Its purpose is to make sure that all stripes of a logical drive are read within a reasonable period of time, thus trying to ensure that the customer's data is available when needed. This process causes the drives to read the data by issuing read-verify commands. By using the read-verify command, the data from the drives is not transferred to the ServeRAID adapter unless an error is detected and reported by one or more drives included in the stripe. If a single drive reports an error within the stripe, the Data Scrub function initiates read commands to all the other stripe unit drives and the data for this single failing stripe unit is recreated by the ServeRAID adapter from the remaining data and parity stripe units.
After recreating this data, the adapter then issues a write-verify command to the drive that reported the error on the read-verify command and writes this recreated portion of the stripe to that drive. After this write completes successfully, this is now a known good stripe, and data scrubbing can continue with the next stripe. In the event that two or more drives report errors during the read-verify portion of the data scrub, the failing stripe will be added to the Bad Stripe Table.
Synchronization is different from Data Scrubbing in that synchronization reads all portions of a stripe, computing parity from the data portions of the stripe and then comparing the computed parity to the parity read from the drive. If the compare fails, the computed parity is then written to the appropriate drive. This process can impact the performance on the controller and, aside from few circumstances when synchronizations are automatically started, is typically user initiated by using ServeRAID Manager or the command line tool, IPSSEND. Synchronization is automatically started in background mode at the time a parity type logical drive (RAID 5, RAID 5EE, RAID50) is created, and at this time each parity portion of every stripe is created from the existing data portions of the stripe and written to the appropriated drive.
IntelliStation Pro:All operating systems listed
System x:All operating systems listed
Older System x:All operating systems listed
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