Detailed info about backups

Detailed info about backups

[Paul Furness]

Hello.

Can anyone give me some pointers for comprehensive information about
tape backups?

A brief bit of background:

I have linux servers running mostly FC2, some RH9 and a couple of RH73.
One of the servers has some autochangers attached - a DLT, an LT01 and
an LTO2. I have scripts which automate which tape to load based on the
day and what's being backed up. Control of the changers is done using
mtx, control of the drives is done with mt, and the backups are done
using xfsdump and dump from remote machines.

The main problem I have is that I seem to be running out of space on the
tape a long time before I've stored anywhere near as much data on there
as the tape should hold - for instance, I'm trying to back up about 150G
on an LTO2 tape which has native capacity of 200G, and it's failing with
a message suggesting the tape is full.

There are a couple of specific things I'm trying to find out; maybe
someone knows the answers to these and can answer me direct :o)    

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?

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.

Is there any way of finding out how much of the tape has been used by a
backup - in terms of actual tape, not in terms of how much data there
was in the backup? I really want to be able to work out how much space
is left on the tape.
 
Answers to any and all of the above questions very much appreciated.

Thanks.

Paul.

Re: Detailed info about backups

[Stephen Samuel]

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.


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