I got these replies by email, which I think clear it up for me. Still not
sure about whether I can start from scratch by erasing the existing
hardware-compressed tapes, and then issuing mt -f /dev/nst0 defcompression 0
in crontab, but I'm going to try anyway:
Software compression is normally recommended when using amanda as it allows
amanda to better estimate how much will fit on a tape. It's also produces
smaller archives - usually. To turn on and off varies for mt implemention,
but if that's what your manpage says. Also is best to either use it or not.
If you switch over you need to physically label the tape etc otherwise
restores won't be able to read the tape if you got the setting wrong to the
tape...
Martin Hepworth
and
You have to define the dump type to use for compression. If by "switching
between the two" you mean changing the configuration between runs, then yes
you can, it will all still be valid but with different performance.
I use hardware compression on servers where, either, they are busy and can
not additionally do the compression,or they have a very large dump to
perform and compression would cause the backup to continue past the backup
window. (this second option is dependent on your network throughput as it
may actually be faster to compress and send the compressed data)I use
software compression where the machine has plenty of time to do compression
or the backup is small.
On my machine, with DLT8000 the following works
:mt -f /dev/nst0 compression off
mt -f /dev/nst0 compression on
Yes after restarting the tape drive it will revert to its default unless you
use a physical switch to disable it.
If you have the spare processing power then switch off the hardware
compression and you will gain a bit of tape space.The already written tapes
will still be valid backups.
Chris Lee
Joe Donner wrote:
>
> Sorry to be bothersome about this, but will the below work?
>
>
> Joe Donner wrote:
>>
>> Thanks for your reply.
>>
>> do you mean that I will see the behaviour you're describing only if I'm
>> reusing tapes with hardware-compressed data? I only have 3 tapes so far,
>> so don't mind erasing them.
>>
>> I find this slightly confusing. My understanding at this stage is that
>> as long as I start with "fresh" tapes, and then use mt -f /dev/nst0
>> defcompression 0 to switch off hardware compression, I should be OK from
>> then onwards? Surely the argument about the tape drive switching into
>> compression mode (when fed a hardware-compressed tape) then also works
>> the other way round, i.e. when a tape drive has hardware compression
>> enabled, and is fed a non-hardware compressed tape, then it won't enable
>> hardware compression?
>>
>> I see that you're saying use stinit, but I've had a look at that and at
>> this stage it will only add more complexity to (for me) a rather complex
>> situation.
>>
>> If I could get away with erasing 3 tapes, using mt -f /dev/nst0
>> defcompression 0 (maybe add that into crontab for good measure), and it
>> all works reasonably well, then I'll be happy for now.
>>
>> Will appreciate your thoughts and thanks a lot for being so helpful.
>>
>> Joe
>>
>
>
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