Re: Detailed info about backups

[Stephen Samuel <samuel@bcgreen.com>]

Paul Furness wrote:

> What does "block size" mean on a tape (DLT and LTO). Is it the same as
> old disks, where each file fills up at least one block, even if it's
> much smaller than a block? If I make this smaller, will more small files
> fit on the tape?
Block size refers to how much data is written to the tape in one chunk.
This can have an effect on how much data you can write to the tape..

For each 'block' you write to the tape there is a pre-block gap , block header
and then the actual data (possibly followed by a block trailer with CRC/checksum.

If you write your data in too-small chunks, then you can often end up wasting
more space on the pre-gap and header than you do on the actual data.

If you write your data in too-large chunks, the tape may have to physically
pause and wait for the data transfer to complete before writing  the data
when this happens you 'lose streaming', and the tape has to stop, backup
and get back up to speed to write the next block  This slows down the backup
and adds wear & tear to the drive (and sometimes also wastes more space on
the tape)

> 
> How do I get the tape drive to definitely use it's internal compression
> hardware? I don't think it's working and I need it to be. How can I
> check this from linux? Some of the auto-changers have displays on the
> front that specify whether compression is being used, but others don't.

man st
   for more info on the SCSI tape drivers.


-- 
Stephen Samuel +1(604)876-0426                samuel@bcgreen.com
		   http://www.bcgreen.com/~samuel/
    Powerful committed communication. Transformation touching
      the jewel within each person and bringing it to light.