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Re: [Bacula-devel] MaximumBlockSize Problem and Question


On Thursday 06 November 2008 22:47:41 Ralf Gross wrote:
> Alex Chekholko schrieb:
> > On Wed, 5 Nov 2008 16:12:51 +0100
> >
> > Kern Sibbald <kern@xxxxxxxxxxx> wrote:
> > > For writing to tape (providing it is LTO-n) I strongly recommend a
> > > block size not to exceed 256K.
> >
> > Hi Kern,
> >
> > Why do you say that?  Is this thread relevant?:
> > http://www.mail-archive.com/bacula-devel@xxxxxxxxxxxxxxxxxxxxx/msg01246.h
> >tml
> >
> > Also, I would like to corroborate the OP's experiences; I had an almost
> > identical thread about small block size and slow write speed:
> > http://www.nabble.com/LTO-4-performance--td17407840.html
> >
> > In fact, I was unable to get higher block sizes working at all with
> > btape:
> > http://www.adsm.org/lists/html/Bacula-users/2008-05/msg00504.html
> >
> > So I am still stuck at ~22MB/s writing to LTO-4 with the default block
> > size.
>
> I don't think that the blocksize is the problem. I did some tests but
> couldn't get higher results with larger blocksizes. I get 75-85 MB/s
> with the default bs and no additional tuning.
>

That is probably correct, but most likely only because you have a bottleneck 
elsewhere -- probably in one of the points I mentioned.  The speed is always 
capped by the slowest component. Once you remove the other bottlenecks on 
your system, the blocksize will very likely become the bottleneck and then 
you can measure the difference.

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