[Gllug] [bit OT] what block size is best for a tape drive?

[Geo.]

On Monday 08 May 2006 08:35, Mike Brodbelt wrote:
> t.clarke wrote:
> > I have never used the hardware compression on the drives,  as I believe
> > the feature requires suitable software (not available on my machine) to
> > write the necessary command(s) to the drive.  As far as I am aware the
> > drive runs at the same physical speed whether compression is on or off;
> > compression is simply done on the fly and results in more data being
> > written to a given block of tape.
>
> Hardware compression requires no host-side software. Most drives provide
> a means to control it through software - the mt command can toggle
> compression on or off for DAT drives or other compatible hardware.
>
> > The quoted compression rates for drives are highly dependent on the data
> > you are writing.  IF your writing data thats already compressed with
> > gzip/bzip or is inherently compressed like jpegs, hardware compression
> > will most likely do nothing for you  - indeed I have heard say iy might
> > actually slow things down!   Of course the other problem with hardware
> > compression is that you can't (I assume) read the tape with another make
> > of drive?
>
> I believe the DAT compression algorithms are standard, so that all DDS
> drives can read tapes written by similar drives. The compression really
> is pretty transparent - the only problems are that it can slow down
> write rates, and that it makes it impossible for your backup software to
> accurately estimate how much data can fit on a tape. Compressed data is
> also slightly less reliable in the event of tape errors - you tend to
> lose the whole compressed stream, rather than just the affected blocks.
>
> Mike
Mike,
 it has been my experience that even with the same make of  DAT machine (HP) 
but a different model the tapes are not readable if hardware compression was 
used:[
-- 
TTFN
   Caparo
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