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

[Mike Brodbelt]

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

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