Seagate SCSI Interface Product Manual page 238

224 SCSI Interface Product Manual, Rev. J
tions are needed for a typical update write sequence: a read transfer from the device containing protected data,
a write transfer to the device containing protected data, a read transfer from the device containing check data,
and a write transfer to the device containing check data. The array controller also does two internal Xor opera-
tions in this sequence. In contrast, during array controller supervised Xor operations, only three data transfer
operations are needed: a write transfer to the device containing protected data, a read transfer from the device
containing protected data, and a write transfer to the device containing check data. Note that the array control-
ler doesn't do any internal Xor operations. In further contrast, during third party Xor operations, only two data
transfer operations are needed: a write transfer from the array controller to the device containing protected data
and a write transfer from the device containing protected data to the device containing check data. Note that
the array controller only issues one command and does no Xor operations.
Performing the Xor operation in the device eliminates the need for the array controller to perform any Xor oper-
ations. An array controller performs three basic operations that require Xor functionality. These are the update
write, regenerate, and rebuild operations.
For additional information on the use of the Xor commands see ANSI document X3T10/94-111r9.