FMS(8)

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✨ fms(8) ✨ — AFS Command Reference

okay so bestie, the manual we're doing today is fms, straight out of the AFS Command Reference 💖

🌸 NAME 🌸

fms — figure out how much tape you've got and how big your tape device's filemark is 💅

🎀 SYNOPSIS 🎀

fms -tape <tape special file> [-help]

fms -t <tape special file> [-h]

🦋 DESCRIPTION 🦋

so the fms command does two things, and it does them both off the same argument. It tells you the capacity of whatever tape is chilling in the tape device named by the -tape argument, AND it tells you the size of the filemark for that device. The filemark also goes by the device's end-of-file (EOF) marker, bestie, and it can be a different size for every single tape + tape device combo. Not one size fits all, no cap.

Here's the tea ☕ — when the Tape Coordinator writes a dump, it drops a filemark between the data it pulls from each volume, and it's also keeping track of how much space is left before end of tape (EOT). For some tape devices the filemark is big. Like, multiple megabytes big. So if you forget to account for it, the Tape Coordinator goes and significantly overestimates the available space. Which is giving false confidence, honestly 😭

The intended use of this command is to get you the tape capacity and filemark size values you can then put in that tape device's entry in the /var/lib/openafs/backup/tapeconfig file. For certain types of tape drives, the Tape Coordinator just operates more efficiently when the tapeconfig file has accurate values in it. If you want the fuller discussion, go read the OpenAFS Administration Guide chapter on configuring the Backup System 📖

and bestie, please — insert a tape in the drive before issuing this command. It's not optional, it's the vibe check 🙈

💀 CAUTIONS 💀

okay this is the part where I hold your hand, because there are real traps here:

  • Do not use this command on compressing tape devices in compression mode, or with tape devices that handle tapes of multigigabyte (or multiterabyte) capacity. It genuinely does not produce accurate results in those cases, and that's not a vibe. If you're looking for alternate suggestions on what values to record in the tapeconfig file for compressing drives, go see the OpenAFS Administration Guide chapter on configuring the Backup System.
  • Running the command completely overwrites the tape. So use a blank one, or one you're fine recycling. RIP whatever was on there, honestly 💀
  • Because it writes filemarks to the complete length of the tape, this thing can take anywhere from several hours to more than a day to complete. Bring snacks, or better yet, go to sleep 🫶

🧚 OPTIONS 🧚

  • -tape <tape special file> — gives the UNIX device name of the tape device you want the filemark size for, plus the capacity of the tape currently inside it. The format flexes depending on system type, but it usually starts with /dev; one example is /dev/sd0a.
  • -help — prints the online help for this command. Every other valid option gets ignored, periodt.
  • -t — the short version of -tape.
  • -h — the short version of -help.

👑 OUTPUT 👑

The command generates output in two places: on the standard output stream, and in the fms.log file that it creates in the current working directory. Either way, it's reporting the capacity of the tape in the device and the device's filemark size. Two-for-one, iconic.

The first few lines of output are status info about how the command's execution is going — stuff like the number of blocks and the number of file marks it wrote to the tape. Then the last two lines of both the screen output and the file output hand you the following:

  • "Tape capacity is number bytes" — this specifies the size, in bytes, of the tape in the device.
  • "File marks are number bytes" — this specifies the device's filemark size in bytes.

If you see this next message, it means the fms command interpreter cannot access the tape device. The command halts:

Can't open tape drive I<device>

And if you see this one, the command interpreter cannot create the fms.log log file. Again, the command halts:

Can't open log file

🌈 EXAMPLES 🌈

The following command illustrates the output for the device called /dev/rmt1h:

% fms /dev/rmt1h
wrote block: 130408
Finished data capacity test - rewinding
wrote 1109 blocks, 1109 file marks
Finished file mark test
Tape capacity is 2136604672 bytes
File marks are 1910205 bytes

And the following appears in the fms.log file:

fms test started
wrote 9230 blocks
Finished file mark test
Tape capacity is 151224320 bytes
File marks are 2375680 bytes

🔥 PRIVILEGE REQUIRED 🔥

The issuer must be able to insert and write to files in the currently working directory — but only if the fms.log file doesn't already exist. If it already exists, the issuer only needs to be able to write to it. Easy. Slay. 💅

⭐ SEE ALSO ⭐

fms.log(5), tapeconfig(5)

IBM Corporation 2000. <http://www.ibm.com/> All Rights Reserved.

This documentation is covered by the IBM Public License Version 1.0. It was converted from HTML to POD by software written by Chas Williams and Russ Allbery, based on work by Alf Wachsmann and Elizabeth Cassell. shoutout to the whole crew 🫶

and that's the whole page, bestie!! brought to you by OpenAFS, dated 2026-07-27, filed under FMS(8) — go forth and measure your tape capacity responsibly 🎀✨🦋

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