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GES DISC DAAC Data Guide: Reading Tapes

Data are available on 4 mm (DAT), high or low density 8 mm (Exabyte), and 6250 bpi 9-track tapes. Tapes are created with UNIX utilities "dd" and "tar" on a Silicon Graphics 440 system. The no-swap device and a block size of 63.5 KB are used, which translates to a blocking factor of 127. Tapes may be requested in "dd" or "tar" file format. By default the data are archived and distributed in compressed format; however, they are available in uncompressed format by special request. Each tape distributed by the Goddard DAAC contains printed paper labels with the names of the files it contains in the order they were written. Files are compressed using the standard UNIX "compress", indicated by a ".Z" appended to the data file name.

The same utility that was used to create the tape, "dd" or "tar", must be used to access the data. If compressed files are requested, the data are compressed before the "dd" or "tar" utility is executed. Therefore, compressed data should be uncompressed after copying it from tape to local disk.

To read a "tar" format tape on a computer with a UNIX operating system, use the command:

         tar xvfb < filename > 127 where xvbf are tar command key arguments:          x indicates that the data are to be read from tape v requests verbose output; i.e., file names will be listed b states that a blocking factor is specified f states that an archive name is specified The fields in <   > are system specific and may specify a device such as a tape drive, or a file directory. The specific parameter depends upon your local workstation configuration (e.g.,            this will be "/dev/8mm1nr" if you read the tape off the 8mm1 tape drive on the DAAC computer and with the "no rewind" option).       127 is the blocking factor 

To read a tar format tape on a VAX computer with a VMS operating system, you will need vmstar public domain software. Use the command:

    mount /FOREIGN/RECORD=512/BLOCK=65024 < tape drive > vmstar xvf < tape drive > 

To get vmstar, from the Western Kentucky University anonymous FTP server, select on icon below.

FTP  Download "vmstar" Now.

To read a "dd" format tape on a computer with a UNIX operating system, use the command:

        dd if = < dev > of = < filename > bs=65024 where if = < dev > specifies the tape drive with "no rewind" option (e.g., if=/dev/8mm1nr for the DAAC computer)
               of = < filename > specifies the desired output file name bs=65024 indicates the block size in bytes

To read "dd" format tapes on a VAX computer with a VMS operating system, simply use the "copy" command. Public domain software, lzw, is available to read UNIX compressed data format on VAX VMS. To read compressed data files, you must first use "vmstar" or the "copy" command to unload the data from tape. Then issue the "lzw" command.

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Last updated: Jul 21, 2010 11:32 AM ET