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Re: [Bacula-devel] MaximumBlockSize Problem and Question


On Friday 07 November 2008 10:55:57 Ulrich Leodolter wrote:
> On Thu, 2008-11-06 at 22:20 +0100, Kern Sibbald wrote:
> > On Thursday 06 November 2008 22:02:26 Alex Chekholko wrote:
> > > 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?
> >
> > It is my experience with my own testing, and I asked the Quantum
> > technical gurus about using large blocks. They think it is not a good
> > idea at all using values larger than 512K and prefer as I do values
> > around 256K.  The bigger it is the more likely the tape writing will get
> > out of phase and IMO the gain is not worth the possible problems.
> >
> > > Is this thread relevant?:
> > > http://www.mail-archive.com/bacula-devel@xxxxxxxxxxxxxxxxxxxxx/msg01246
> > >.htm l
> >
> > Possibly.
> >
> > > 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.
> >
> > There are a lot of other parameters to consider, but I cannot give such
> > advice any more -- no time:
> >
> > - Up the block size to 128K or 256K and verify that it is being written
> > that size.
>
> Hi,
>
> Be warned!  Changing this parameter will deny access to existing
> backup data.

>
> > - Increase the Maximum File Size to about 5GB.
>
> Are existing data still accessible when increasing
> Maximum File Size ?  i hope so

Changing the Maximum File Size is completely upwards/downwards compatible, 
with the reservation that if you make it too small you will incur increased 
overhead, and if you make it larger, it may take longer to recover files. 

Changing the tape block size can create compatibility problems.

Regards,

Kern

>
> BR
> Ulrich
>
> > - Use only *very* fast RAID disks running at 15,000 rpm.  Maybe soon we
> > can use SSD.
> >
> > - Get only the best disk and tape controllers.
> >
> > - Get good quality, big, fast hardware -- you need hardware experts to
> > get lots of speed.
> >
> > Then you need to do the same kinds of tuning of your network.
> >
> > Regards,
> >
> > Kern
> >
> >
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