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
Jon LaBadie wrote:
>
> On Wed, Jul 05, 2006 at 02:42:14AM -0700, Joe Donner (sent by Nabble.com)
> wrote:
>>
>> Thanks to everyone for your replies.
>>
>> Ok, so what I really wanted to do was to use software compression, and
>> not
>> hardware compression.
>>
>> Cyrille's suggestion sounds like what I need, but again I'm not sure
>> whether
>> using this command will mean that compression is turned off
>> "permanently".
>> I'll look into the stinit command in the meantime...
>>
>> Now I'm wondering about the tapes I've already used while hardware
>> compression was still on. Obviously the tape drive will need to feed
>> those
>> tapes through its decompression mechanism to read them again. If I use
>> the
>> command as suggested by Cyrille, will that mean that the used tapes
>> become
>> unreadable, or that you have to manually turn compression on and off
>> (I've
>> read the the compression command overrides the defcompression one for the
>> currently loaded tape)?
>>
>> Thanks.
>>
>> Joe
>>
>>
>> Cyrille Bollu wrote:
>> >
>> > owner-amanda-users AT amanda DOT org a ?crit sur 04/07/2006 13:17:46 :
>> >
>> >>
>> >> Dear all,
>> >>
>> >> I just want to clarify something about compression:
>> >>
>> >> My understanding is that you can use either software or hardware
>> >> compression, or can switch between the two if needed.
>> >>
>> >> What is generally, in your experience, the best of the two to use?
>> >>
>> >> To switch off hardware compression, I believe I should use:
>> >> mt -f /dev/nst0 compression 0
>> >>
>> >> Does this command switch it off permanently until you use the
>> > following?:
>> >> mt -f /dev/nst0 compression 1
>> >>
>> >> or will hardware compression switch back on, say, after a reboot?
>> >
>> > AFAIK, there's another option.
>> >
>> > from "mt" man pages:
>> >
>> > defcompression
>> > (SCSI tapes) Set the default compression state. The
>> value
>> > -1
>> > disables the default compression. The compression state
>> > set by
>> > compression overrides the default until a new tape is
>> > inserted.
>> > Allowed only for the superuser.
>> >
>> > so one should use:
>> >
>> > mt -f /dev/nst0 defcompression 0
>> >
>
>
> Some posters have noted that in their environments when the tape drive
> reads data that was HW compressed, it resets itself to HW compression.
> This greatly impacts your situation if your drive does the same thing.
>
> The first thing most (all?) amanda commands in preparation for writing
> is open the tape, read the tape label to confirm it is an amanda tape
> (and the expected one), then close the tape before reopening it again
> for writing. Thus, unless you can interpose a "turn off HW compression"
> between the close and reopen, the drive will be in HW compression mode
> again. The only way(s) to force the HW compression off at each opening
> is with stinit or perhaps Cyrill's defcompression.
>
> Gene Heskett has posted a number of times the steps he uses to ensure
> tapes previously used with HW compression won't trick the drive. It
> basically is rewind, close the drive, force HW compression off, use
> dd to write a lot of data to the beginning of the tape (20-100MB IIRC).
>
> If needed you can check the list archives at yahoo or marc for his posts.
>
> --
> Jon H. LaBadie jon AT jgcomp DOT com
> JG Computing
> 4455 Province Line Road (609) 252-0159
> Princeton, NJ 08540-4322 (609) 683-7220 (fax)
>
>
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