On Oct 23, 2014, at 3:48 PM, Jon LaBadie <[email protected]> wrote:
> On Thu, Oct 23, 2014 at 10:34:38AM -0400, Gene Heskett wrote:
>> On Thursday 23 October 2014 01:28:01 Tom Robinson did opine
> ...
>>
>> If you are feeding the tape device compressed files, and the drives
>> compressor is enabled too, this will quite often cause file expansions on
>> the tape itself. The drives compressor, because it is intended to handle
>> the compression on the fly, is generally not sophisticated enough to do
>> any further compression and will add to the datasize, expanding what
>> actually goes down the cable to the drives heads.
>
> Tom is using an LTO drive (-5 I think). Most modern tape
> drives, including all LTO's do not exhibit the bad behavior
> of the DDS drives with their run-length encoding scheme.
>
> IIRC, they have enough cpu smarts and memory to first
> collect the data in memory, try to compress it to another
> another memory buffer, and if it is enlarged the block
> is saved "uncompressed".
>
> Note, instead of a flag at the start of the tape indicating
> compressed or uncompressed, there is a flag for each tape
> block.
>
> jl
> --
> Jon H. LaBadie [email protected]
> 11226 South Shore Rd. (703) 787-0688 (H)
> Reston, VA 20190 (609) 477-8330 (C)
Oooo — so with LTOx tapes one can forget that “turn off hardware compression
by
rewriting start of tapes” bit?
I’ve just moved to LTO2 tapes, and will be going to LTO5 tapes as soon as
my
new hardware is setup. This would be good to know!
Deb Baddorf
Fermilab
