Solving Incorrect Block Size Problem In Backup Tapes

April 1st, 2009

Its common phenomena to take Unix tape backups on one system and restore on another. Moving tapes to another site for safety sake is also called offsite backup. The problem often occurs while restoring tape data on another server. The tape backup access is very much dependent upon the block size used on the Unix server. For example on one unix server the block size can be 2048 Bytes and on another it may be 1024 Bytes.

Normally the tape backup is written in 512 Bytes block size. Now, suppose the target unix server is using the block size other than this one e.g 2048 Bytes, the tape becomes temporarily unavailable. The reason is block size mismatch.

You get something like this;

# tar -tvf /dev/rmt0

tar: 0511-193 An error occurred while reading from the media.

There is an input or output error.

This problem can be solved in two ways:

  1. tcopy command in Aix.

  2. The dd commanad, common to almost every unix system.

The tcopy command

Let’s say the tape device is /dev/rmt0, then the output of the following command will give the size of tape block size.

tcopy /dev/rmt0

tcopy : Tape File: 1; Records: 1 to 8796 ; size:1024

tcopy : Tape File: 1; End of file after :8796 records; 9474628 bytes

tcopy : Tape File: 2; Records: 1 to 3400 ; size:1024

tcopy : Tape File: 2; End of file after 3400 records, 76890 bytes

….

size:512 specifies tape block size.

The dd command

Simply using dd command to read one block of tape device will give the tape block size.

For example: dd      if     /dev/rmt0    bs=128k      count=1 | wc -l

The above command gives the size of tape block size.

Now, once you know the tape block size, you can change the tape block size. For Aix its give below:

#chdev -l /dev/rmt0 -a block_size=1024

Now attempting, below command will be successful.:

# tar -tvf /dev/rmt0

-rw-rw-r– 0 0 1327 Apr 01 12:18:17 2600 /var/adm/ras

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