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What are CRC and underrun errors?

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Applies to

Data ONTAP 7 and earlier

Answer

CRC errors
CRC errors exist in the data payload of frames that circulate through a Fibre Channel-Arbitrated Loop (FC-AL). The errors are detected by devices inside the loop when Data ONTAP writes to a disk-for example, a disk or ESH module. The error-detecting device generally is not responsible for the errors. 

When CRC errors are detected, Data ONTAP retransmits the affected data. Usually, these errors are transient, so a retransmission will clear the problem. Data ONTAP attempts to repath and retry the I/O operation three times. If three retransmissions are unsuccessful, Data ONTAP will fail the disk. 

Note: When troubleshooting CRC errors, keep in mind that the device causing the CRC error exists somewhere before the disk reporting it in the loop. Use the "fcadmin link_stats" and "device_map" commands to see what comes before the failed disk in the loop.

Underrun errors
Underrun errors happen when a read operation, requested by the initiator, gets corrupted by a device on the path between the disk (to be read) and the initiator (filer adapter). When troubleshooting increasing underrun errors, examine devices after the one reporting the error; most of the data flow that could get corrupted will be reads from the disk to the initiator.

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