On 31 January 2018 at 10:26, Alan Brown <a.br...@ucl.ac.uk> wrote:
> On 31/01/18 10:14, Ben Laurie wrote:
> > I also have that on my LTO3 drive.
>
> You should test this using the fill command in btape (which will test
> with uncompressible data). if the block size is set correctly you're
> going to achieve very close to the published throughput.
>
> Please note that 250MB/s for LTO4 is faster than the uncompressed speed
> of the tape, but _slow_ for compressible data.
>
> Experiments here found the optimum size to be at least 256kB. Mine's set
> to 1MB.
>
Optimum in terms of transfer speed?
>
> Also note that block size is not the same as the tape drive buffer size.
>
> Early drives have around 100MB buffer and LTO4 onwards have at least
> 600MB buffer.
> I suspect that's been increased again in LTO8 but haven't got my hands
> on one to interrogate.
>
>
> Apart from write speed, smaller blocks mean you will write less data per
> tape due to the ratio of block headers to data, and smaller blocks have
> a significantly lower restore speed.
>
One of the surprising things that happened when I switched from an LTO2 to
an LTO3 drive is that I now sometimes get 1TB on an LTO tape(!). I have
been assuming that this is because there's more compression history, but
haven't had a proper explanation yet. Mostly, BTW, tapes fit 300 or so GB.
Anyway, suppose I find I should be using a different block size, does that
cause me any trouble wrt existing tapes?
>
>
>
> >
> >
> > On 21 December 2017 at 23:08, Dan Langille <d...@langille.org
> > <mailto:d...@langille.org>> wrote:
> >
> > Hello,
> >
> > I'm using Bacula 7.4.7 on FreeBSD 11
> >
> > What Bytes/block are you using for your LTO4 tapes?
> >
> > I ask because I just noticed:
> >
> > ###
> > Device tape is "LTO_0" (/dev/nsa0) mounted with:
> > Volume: 000034L4
> > Pool: FullsLTO4
> > Media type: LTO4
> > Slot 38 is loaded in drive 0.
> > Total Bytes=20,127,937,536 Blocks=312,002 Bytes/block=64,512
> > Positioned at File=20 Block=2,003
> > ###
> >
> > What are you using and why?
> >
> > Thank you.
> >
> > --
> > Dan Langille - BSDCan / PGCon
> > d...@langille.org <mailto:d...@langille.org>
> >
> >
> >
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