[clug] LTO-4 drive, turn on compression? [SEC=PERSONAL]

[Duncan Roe]

On Thu, Aug 13, 2009 at 06:01:05PM +1000, Daniel Pittman wrote:
> Chris Smart <mail at christophersmart.com> writes:
> > 2009/8/13 Roppola, Antti - BRS <Antti.Roppola at daff.gov.au>:
> >> On our Exabyte drives, the compression settings showed up as devices (on
> >> Solaris anyway).
> >>
> >> So something like.... ??tar cvf /dev/nrst7
> >
> > Thanks Antti. Someone else suggested this offline, however Linux only
> > offers two devices AFAIK, rewinding and non-rewinding.

Hi Chris,

stinit gave me 4 devices x 2 (4 rewinding, 4 not). I think it configures the
Linux driver to turn compression on or off, and to set block size, depending on
which drive you use.
> >
> > Nevermind, I'm just doing standard old tar across multiple LTO-3
> > tapes.. sure isn't fast. Wrote one tape last night and has been going
> > on the second since 8am!
>
> Ah.  You used too small a block size, or you can't feed the streamer so it is
> having to stop and start all the time.
>
> You need at least a 64K block size to stream on an LTO2, from my testing, and
> better 256K.  With LTO3 improving the streaming speed I would guess that 256
> or 512K blocks would be a good strategy.
>
> You also want to make sure you can feed at least 20MB/second to the device,
> although that probably varies a bit from vendor to vendor.
>
Hi Daniel,

A very long time ago, I read in an (early) Solaris newsletter that 63K was
optimum (-b 126 to tar). I did some timing tests of that, larger and smaller
values and indeed it was true - 63KB was best. But I've had a succession of tape
drives and that may have been on the old DDS (i.e. DDS-1). Might still be worth
checking performance of power of 2 - 1 against power of 2 though.

Cheers ... Duncan

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