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Re: [Bacula-devel] MaximumBlockSize Problem and Question


Kern Sibbald schrieb:
> 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.

I didn't want to compain, just show the org. poster that his 22 MB/s
are likely not a bs issue.

That being said, I started a thread on the user list a while ago where
I aked what throughput people are getting when writing to tape. Nobody
involved in this thread got higher numbers than 80-85 MB/s for a
single job.

I backup directly over GbE interfaces from a RAID that can deliver 350
MB/s. Network throughput is ~115 MB/s and we are mainly backing up 
very large files. Setting Maximum Network Buffer Size and other
options didn't make any difference to the default values. Spooling
dind't help, in fact it was slower during despooling because the spool
disks are slower than the GbE network + the RAID of the client.

I know that the developer list is not the perfect place for this, but
maybe someone here can share his numbers? What overhat will bacula add
to the transferred data?

Thanks, Ralf




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