To write files to a tape drive with a visible progress bar, pipe the output of tar through pv. Use the command tar -cvf - /path/to/data | pv > /dev/nst0. This approach leverages standard Linux utilities to provide real-time feedback during the backup process without requiring specialized proprietary software.
Understanding the Components
The solution relies on three core Linux commands working together in a pipeline. Each component has a specific role that ensures data integrity and user visibility during the long-running tape write operation.
The Tar Command
tar is the standard utility for creating archive files. In this context, it reads the source directory or files and outputs the raw stream of data to standard output (stdout). The flags used are critical:
- -c: Creates a new archive.
- -v: Verbose mode, which lists files as they are processed. This provides immediate visual confirmation that the command is active.
- -f -: Specifies the file name. Using a hyphen (
-) tellstarto write the archive to stdout instead of a regular file. This is essential for piping the data to another program.
The PV Utility
pv (Pipe Viewer) monitors the progress of data through a pipeline. It provides statistics such as transfer speed, elapsed time, and percentage completion. Without pv, writing to tape can appear frozen because tape drives operate at variable speeds depending on block sizes and hardware state. pv gives you the confidence that the system is still working.
The Tape Device
Tape devices in Linux are typically represented by special character devices in the /dev directory. The most common device for SCSI tape drives is /dev/nst0. The 'n' stands for "no rewind," meaning the drive will not automatically rewind to the beginning after an operation, allowing you to append multiple archives to the same tape. Other variations include /dev/st0 (with auto-rewind) or numbered instances like /dev/nst1 if multiple drives are present.
Executing the Backup Command
Constructing the final command requires chaining these components correctly. The general syntax follows the pattern: source_command | pv > destination_device.
tar -cvf - /path/to/backup/source | pv > /dev/nst0When you run this command, tar begins reading files from /path/to/backup/source. As each file is compressed into the stream (if compression were enabled), it passes through pv. pv displays a progress bar on your terminal screen. Finally, the data stream is written directly to the tape device /dev/nst0.
If you need to exclude certain folders, refer to our guide on Shell Command to Tar Directory Excluding Certain Folders. You can add exclusion flags to the tar portion of the pipeline before the pipe symbol.
Verifying the Tape Content
After the write operation completes, it is prudent to verify that the data was written correctly. You can list the contents of the tape without extracting it using the following command:
tar -tvf /dev/nst0This command uses -t to list the table of contents, -v for verbose details, and -f /dev/nst0 to specify the tape device. If the listing matches your expectations, the backup was successful. Note that some older tape drives may require you to manually rewind the tape before performing a read operation if it did not auto-rewind after writing.
Optimizing Performance
Tape drives often perform better with larger block sizes. You can influence the block size used by tar using the --blocking-factor option. For example, adding --blocking-factor=64 creates 32KB blocks, which reduces the number of start-stop operations on the tape, increasing throughput.
However, be aware that changing block sizes might affect compatibility with other systems reading the tape. Always test with your specific hardware. Additionally, ensure that no other processes are accessing the tape device simultaneously, as this can cause I/O errors. If you are managing multiple sync processes, consider reviewing Limiting Resource Usage in Sync Processes for Off-Site Backups on Older Machines to prevent resource contention.
Common Pitfalls
Permission Denied: Ensure you are running the command as root or a user with write permissions to the tape device. Typically, tape devices are owned by root or the tape group.
Device Busy: If you receive a "device busy" error, check if another process is holding the device open. Use fuser /dev/nst0 to identify any interfering processes.
Missing PV: If pv is not installed, you can install it via your package manager (e.g., sudo apt install pv or sudo yum install pv). Alternatively, you can use dd with status options, but pv offers superior visual feedback for interactive monitoring.
Frequently Asked Questions
Can I compress the data while writing to tape?
Yes, you can add the -z flag to gzip compress the data on the fly. Use tar -czvf - .... However, note that compression adds CPU overhead and may slow down the initial processing speed compared to uncompressed writes.
How do I rewind the tape after writing?
If you used /dev/nst0 (no rewind), you must rewind it manually before reading or erasing. Run tapecontrol -w /dev/st0 or simply use the mt command: mt -f /dev/nst0 rewind.
Why is my progress bar stuck at 0%?
This usually happens if tar encounters a permission error on a specific file and stops, or if the tape drive is offline. Check the verbose output above the progress bar for error messages indicating which file caused the halt.
Is rsync better than tar for tape backups?
Rsync is excellent for network transfers and incremental updates, but tar is generally preferred for full disk images and tape storage due to its ability to handle large contiguous streams and preserve all file attributes efficiently. See our article on Is rsync idempotent? to understand its limitations for this use case.
What happens if the tape runs out of space?
The tar command will continue until the end-of-media is reached, then throw an I/O error. pv will also stop updating. You should monitor the available space on the tape beforehand or use scripts to detect end-of-tape signals programmatically.