Linux: Tape backup for a home network

Introduction

In another article, I have discussed why tapes are still the most efficient form of backup. In this article, I detail how I used tapes in a Linux based home network for backup purposes, using a Seagate/Certance STT20000A IDE ATAPI Internal Travan TR-5 Tape drive. There are also several resources listed.

Read below for more ...

Background and Historical Notes

I have been backing up my personal stuff on tape for about 15 years. I used to make copies of my home directory (on UNIX, at work) to a QIC tape that had a whopping 60 MB capacity, running on an NCR TOWER (Motorola 680x0 CPU). Later, I would use 150 MB QIC tapes on the same architecture.

I then migrated to the same drives but on NCR System 3000 (Intel x86 CPU), and had to do byte swapping conversion on the data:

dd if=/dev/rmt/c0s0 conv=swab | cpio -icvmuld

Since QIC drives were expensive, this was out of my reach at home. But there were plenty of those at work, and my stuff was mainly on my machine at work. This high cost factor alone contributed to my being delayed in running UNIX/Linux at home for years.

When I started using Linux at home, I bought an HP Colorado 2.5GB/5GB Travan IDE tape drive. It worked well and I backed up stuff on it when my hard drive was just 2GB. I used several tapes, and backed up every week or two, normally overnight.

The tape drive was recognized as an IDE Tape, so the device was /dev/ht0, so in the dmesg output, or in /var/log/messages you should see something like:

ide-tape: hdd <-> ht0, 600KBps, 14*26kB buffer, 2600kB pipeline, 310ms tDSC, DMA

A Real Life Example

In 2000, my home was broken into and the PC stolen! Those tapes came in handy with all my data on them. I only lost about a month's worth of data.

I bought a used Seagate 10GB/20GB Travan IDE tape drive off someone selling it on eBay for about 100$US, including several used tapes. I restored my home directory and everything was good to go. If I had bought another hard disk and copied stuff to it, all my data would have been gone forever! Remember that next time you say "buy another disk!"

All the computers on the home network share the server's drive using either Samba (for Windows clients) or NFS (for Linux clients). Each user is responsible for making sure that their files end up on the server, otherwise, they are not backed up.

I use a set of 6 tapes for weekly backup and 3 for monthly backups, all using cpio. The monthly ones are kept offsite, at a friend/co-worker's house. Even with removable hard disks, you can't justify the cost of 9 hard disks!

The Linux cron facility is used to start the backup in the early hours of the weekend. I even have another cron job the day before to check that the tape is inserted in the drive and that it has been rewind to the beginning of tape. The entire process is unattended, except for an email telling me the backup is done, and to change the tape, or to remind me of the monthly backup.

My backup is about 7 GB now for the home server, which contains my kids homework, my files, digital pictures, CVS repository, ..etc. So there still room on the tapes.

Configuration

In order to configure it with Linux, you need to specify the kernel argument hdx=ide-scsi (where x is the letter of the IDE position for that drive). You can specifiy this argument in the lilo or grub configuration file.

You will know that the tape is correctly installed and that it is recognized by the Linux kernel, when you run the dmesg command or view the /var/log/messages log, and see the following:

First a message that the IDE driver has recognized the drive.

hdd: Seagate STT20000A, ATAPI TAPE drive

Then, you should see the SCSI over IDE layer loaded, and it should recognize the tape as well:

SCSI subsystem initialized
scsi0 : SCSI host adapter emulation for IDE ATAPI devices
  Vendor: Seagate   Model: STT20000A         Rev: 8.25
  Type:   Sequential-Access                  ANSI SCSI revision: 02

Finally, you should see the SCSI Tape driver loaded:

st: Version 20040122, fixed bufsize 32768, s/g segs 256
Attached scsi tape st0 at scsi0, channel 0, id 0, lun 0
st0: try direct i/o: yes, max page reachable by HBA 163838

The tape device is now known as /dev/st0 to the tape programs.

You can also use /dev/nst0 if you do not want the tape to rewind on close. This is useful if you want to be creative and "append" backups on a tape.

Note: You may see the following messages in the output of dmesg or in /var/log/messages. Do not be alarmed. There is no ill effect from there, and they are harmless:

Jul 14 12:53:36 zzzz kernel: st0: Error with sense data: Current st0: sense key Illegal Request
Jul 14 12:53:36 zzzz kernel: Additional sense: Invalid command operation code

Backup and Restore Commands

You can then use the following commands:

To get the tape status, do the following:

mt -f /dev/st0 status
SCSI 2 tape drive:
File number=0, block number=0, partition=0.
Tape block size 512 bytes. Density code 0x47 (TR-5).
Soft error count since last status=0
General status bits on (41010000):
 BOT ONLINE IM_REP_EN

Note the status bits at the end of the output. These are useful, but are often cryptic. Here is an explanation of the most important ones:

If you do not have a tape cartridge inserted in the tape drive, you will see:

DR_OPEN IM_REP_EN

If you have a tape cartridge in the tape drive, but the write protect tab is on, you will see:

BOT WR_PROT ONLINE IM_REP_EN

In order to rewind the tape, do the following:

# Rewind the tape
mt -f /dev/st0 rewind

For actual backup, I do not use dump since it only dumps one filesystem, and does not cross the boundary. Therefore I will need two tapes to backup my system, which is kind of a waste. I could actually make the backups back to bak, but this complicates things if I am looking to read info from the tape, specially in the panic of a recovery situation. It is best to backup everything using cpio, with relative pathnames, and then restore whatever you need to wherever you need, even if partitions are different.

Therefore, I prefer to use cpio. Even tar is a bit primitive compared to cpio, since it has many nice features, such as preserving the modification time.

To backup, use the following command:

# Backup a directory using cpio crc header format
cd /home
find . -print | cpio -ovH crc -O /dev/st0

The -H crc option specifies that the backup will be in a portable format that can be read on other UNIX machines, and not only Linux.

If you are doing a full system backup, then you have to exclude some directories, such as /proc, /dev and /mnt.

To list the tape contents, you can do:

cpio -itv -I /dev/st0

To restore the tape contents, you can do:

cpio -icvmuld -I /dev/st0

Check the appropriate manual pages for the mt and cpio commands for more details (links below in the Resources section). You may also want to check the dd manual page as well, since you can do some neat tricks with it.

So, I will stay with tape for the forseable future, until optical technology catches up with disk size increases.

Resources and Links

• Thanks to LinuxQuestions, the manual pages for Linux commands are available online, in HTML format. Here are the pages for
  • cpio
  • tar
  • dump
  • mt
  • dd
• How to use your tape drive under Linux from the Swiss Federal Institute of Technology in Zurich. This is a good tutorial on tapes and Linux. Although it assumes you have SCSI and DAT drives, much of it will apply to IDE tapes using SCSI emulation. Ignore the parts about ejecting the tape, changing density and compression, and everything will work.
• White Paper on DLT Tapes on Linux systems. This document discusses the use of Digital Linear Tape (DLT) under Linux. Although their stated goal in the document is about DLT, most of the document applies to any other tape device on Linux. It details how to use mt, tar, cpio and dump.
• Linux Tape Backup page, with emphasis on the dump command.
• An older article on Travan IDE tapes with Linux.
• An article on using Travan IDE tapes with Arkeia, a backup utility.
• Linux Tape Certification is a web site listing many drives that are supported by Linux, and their performance, and feature comparison.
• The drive I currently use is a Seagate STT20000A 10GB/20GB Travan TR-5 IDE Internal. It is now marketed by Certance, and also available from Dell.
• A very useful article on how to use the dd command for various things, including reading a tape with a bad spot, byte swapping, character set translation, ...etc.
• Linux Backup and Restore Procedures from the Linux Admin Made Easty document. Focuses on using tar and KDat.
• UNIX Data Backup HOWTO by Tariq Nazir. This is UNIX specific, so /dev/rmt/xxx should be /dev/st0, and ufsdump should be just dump if you use that on Linux.
• How to use tapes on UNIX. This document is a bit dated (1999), but contains a wealth of information on tape devices and backup commands. Most of that can be applied directly to Linux.
• It is always important to define a media rotation scheme for tapes. Two popular methods are Grandfather, Father, Son and Towers of Hanoi.