Configuring the density of a tar device

kaplan71 · 06-26-2007, 03:24 PM

Hi there --

I have been running a nightly tar backup script to a DDS-3 tape device for some time. Recently, I noticed the amount of data that is being backed up has exceeded the storage capacity of the DDS-3 device. I replaced the drive with a DDS-4 device capable of 40GB storage.

Last night's backup onto the new device failed supposedly because there was not enough space left on the device. Listed below is the output to the log file:

SCSI 2 tape drive:
File number=0, block number=0, partition=0.
Tape block size 0 bytes. Density code 0x26 (DDS-4 or QIC-4GB).
Soft error count since last status=0
General status bits on (41010000):
BOT ONLINE IM_REP_EN
tar: Removing leading `/' from member names
tar: /var/run/avahi-daemon/socket: socket ignored
tar: /var/run/dbus/system_bus_socket: socket ignored
tar: /var/run/sdp: socket ignored
tar: /dev/st0: Cannot write: No space left on device
tar: Error is not recoverable: exiting now

I ran the du and ncdu commands, and both indicated the amount of data that is to be backed up is no more than 27GB. Is there a way to configure the density of the device prior to or during the tar backup? The current syntax in the script is as follows:

sh -c 'tar -cf /dev/st0 $(cat /root/backup_list.txt)'

If the above is not the way to do it, the other approach I thought of was modifying the tar line shown above to read as follows:

sh -c 'tar -czf /dev/st0 $(cat /root/backup_list.txt)'

jschiwal · 06-26-2007, 04:12 PM

You could use compression in the tar command with the -j -z options. The tar manpage lists the option -[0-7]lmh and says -[0-7]lmh changes the density and(sic) the drive. I assume that -m means medium density, and -h means high density.

What is the device (as in /dev/st???) that this tape player uses. There may be a corresponding readme file for the device in the linux Documentation directory (of the kernel source) that lists device options. ( You could look at the kernel source on your personal laptop or on the web instead of installing the kernel source on the server ) Sometimes documentation in the kernel provides additional information or at least more authoritive information, since it is more current and matches your kernel).

marozsas · 06-26-2007, 04:34 PM

I don't know if the following applies to you, but I realize that when I backup already compressed files to a dat drive which has hardware compression enabled, the resulting data is bigger than the original data.

This is because the hardware compression algorithm is dumb and it can not detect that data is already compressed.

anyway, try to disable hardware compression in your drive if you are backing up compressed data (JPEG, GIF, PNG, zip, gzip, etc)

to get hardware compression status:

Code:

root@bigslam:~>mt -f /dev/nst0 datcompression 1
Compression off.
root@bigslam:~>

to set data compression off, use count=0

Code:

root@bigslam:~>mt -f /dev/nst0 datcompression 0
Compression off.
root@bigslam:~>

any value other than 0|1 enables hardware compression.

kaplan71 · 06-27-2007, 10:15 AM

Thanks for your reply. I believe I discovered the problem to the backups. I manually ran the backup today using the following command syntax:

tar -czvf -7 /dev/st0 $(cat /root/backup_list.txt)'

During the backup I noticed the / directory eventually got filled to capacity. Shortly after that, I encountered the error message that I had mentioned in my original e-mail.

I ran the ncdu utility, and tracked down the file in question. It was a file that was associated with the backup. It's name was -7. I removed the file and was able to get free disk space made available.

There is not enough diskspace available in any directory for this file during the backup. Is there a way to do the backup without this file being created? Thanks.

marozsas · 06-27-2007, 10:31 AM

the arguments for tar are in a wrong order. The -f key expect the NEXT argument to be the destination of backup, the tape drive device /dev/st0, not another key....
The correct way should be:

Code:

tar -7 -czvf /dev/st0 $(cat /root/backup_list.txt)

(there is a extra single quote in your cmd line, too)

In your wrong cmd line, nothing was sent to tape. Instead, the backup was created in a file named "-7". That is why you get a big file in your filesystem.

I hope this is clear now.

kaplan71 · 06-27-2007, 12:05 PM

Hi there --

I have run into another problem: I entered in the correct syntax:

tar -7 -czvf /dev/st0 $(cat /root/backup_list.txt)

and the error message I get is:

Options '-[0-7][lmh]' not suppored by *this* tar

The version of tar that I am using is GNUtar 1.16. Has anyone seen this, and know how to get around it? Thanks.

marozsas · 06-27-2007, 12:20 PM

Don't bother yourself with this density option.
I may be wrong, but density option is kind of deprecated now. It was useful on older QIC tapes but with modern DAT drives it is useless.