On Thursday 06 November 2008 22:02:26 Alex Chekholko wrote:
> On Wed, 5 Nov 2008 16:12:51 +0100
>
> Kern Sibbald <kern AT sibbald DOT com> 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 AT lists.sourceforge DOT 
> net/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.

- Increase the Maximum File Size to about 5GB.

- 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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