Definition

LUN Bad Sector Tag is a technology used to tag the uncorrectable bad sector. By examining the sector fault, it reduces the granularity of the damaged area to sector level, thus avoiding service interruption and data loss caused by the fault of the disk.

Purposes

This feature can avoid disk fault or RAID group fault during the reconstruction after the RAID group is degraded and prevent service interruption and data loss.

Benefits

By using the LUN bad sector tag feature, the granularity of the damaged area is reduced and the service reliability is enhanced.

Availability

You do not need to purchase an extra license for LUN bad sector tag.

Principles

If uncorrectable bad track is examined during the reconstruction or ordinary I/O operation when the RAID group is degraded, the LUN bad sector tag feature tags the bad sectors positioned in the bad track. When the host read data in this area afterward, it returns Medium error to the host. When the host writes data to this area, it triggers bad track repair which is performed on this area and nearby position.

This document takes RAID 5 as an example. As shown in Figure 1, under the condition that a member disk is faulty, the RAID group loses redundancy function, and the I/O requests in degraded mode or reconstruction is performed. In this process, when an uncorrectable track on another member disk is examined during I/O operation, if disk fault is reported, the entire RAID group gets faulty and the data on the entire RAID group is lost. By LUN bad sector tag technology, the bad sectors nearby the bad track can be tagged and the data in this area can be ignored. In this way, the I/O requests and reconstruction can be continued, and does not lead to service interruption and data loss. When the host read data in this area afterward, it returns Medium error to the host. When the host writes data to this area, it triggers bad track repair which is performed on this area and nearby position. If the bad track is repaired successfully, the bad sector tag is cleared and normal I/O requests performed on this area are resumed.

Application Scenarios

The application scenarios that has higher requirement on data reliability and lower requirement on data integrity, such as video supervision and database.