About DIV

DIV is supported on Oracle's StorageTek T10000C tape drives. When the DIV mode is enabled on the SAM-QFS file system for the archive copy, the SAM-QFS file system enables DIV in the st driver using the mt DADP command. The st driver then calculates a checksum (hardware or software generated depending on the server platform) for every logical block written by the archiver to the tape. The checksum is checked by the tape storage drive and it must match the data to be written to the tape. The StorageTek T10000C drive will put both the user data and the checksum on the media. If the checksum does not match, an error is returned to the archiver and the archiver does not mark the file as copy archdone. The st driver retries the DIV errors during both the read and the write operation. If the errors are only temporary, then the st driver will successfully transfer the data without returning an error.

SAM-QFS uses the st driver to check the checksum for every logical block read by the stager from the tape. The checksum must match in order for data to be returned to the user. The stager will move to an alternate copy if an error is detected. DIV gives immediate feedback at the block level to guarantee that only good data is written to the tape and only good data is returned from the tape. The StorageTek T10000C drive enables a very fast user data verify option without having to transfer the data back to SAM-QFS. Because the checksum is stored on the media with the user data, it can be verified directly by the tape storage device.

SAM-QFS supports a new tpverify command for the StorageTek T10000C drive which will verify the data written to tape without staging the data. This check can be done periodically to verify that the data on the tape is correct. If the data is not correct, SAM-QFS stages the alternate copy and rearchives the file automatically. Also, the tpverify command will return errors if it detects incorrect data. Another archiver capability is verify-after-write, which causes the archiver to verify the entire file after it is written on tape. The verify-after-write capability ensures that the DIV Cyclic Redundancy Codes (CRC) is correctly stored on tape.

For the Sun SAM-Remote software, in order to use the DIV feature, the div settings in the default.conf file must be enabled for the SAM-Remote server as well as the client. For more information about the SAM-Remote Software, see Chapter 18, Using the Sun SAM-Remote Software.