Networker

Re: [Networker] setting block size

2003-08-11 05:15:09
Subject: Re: [Networker] setting block size
From: Davina Treiber <treiber AT HOTPOP DOT COM>
To: NETWORKER AT LISTMAIL.TEMPLE DOT EDU
Date: Mon, 11 Aug 2003 05:15:20 -0400
Oh dear, so much misinformation in one thread. This subject has been
covered so many times before, yet people are still posting stuff that is
plain wrong. Let's try to set the record straight.

Jose Quinteiro <jquintei AT LEGATO DOT COM> from Legato, who should know better
and should have read the release notes and been on all the training posted:

> Block size is set automagically by Networker based on device type.  As
> long as the devices are configured to be the same type, the block size
> should be the same regardless of platform.

> The environment variables are used to override Networker's defaults.
> The defaults should be fine.

This is wrong. The default block sizes do vary between platforms for some
device types. This is because of the limitations of certain HBAs and their
drivers.

The same Legato employee then posted with apparent authority:

> There is a registry key that you need to change to get some HBAs to
> write blocks larger than 128K in Windows.  I'm racking my feeble brain
> to try to remember the details.  Google search was fruitless.  I'm not
> near the co-worker who is incredibly knowledgable on this subject, but
> I'll email him.

> FWIW, the "Not enough space" messages typically mean the tape is blank,
> but you read a label sucessfully.  Any errors in your system log?

He of course should have known that "not enough space" frequently means
that there is a block size mismatch. He also should know his employer's
stance on employees posting to this list with a Legato email address -
especially when what they are posting is garbage.

Carter Fowler's posting is much nearer the mark; it is often necessary to
modify the scatter/gather parameters for the SCSI card you are using in
order that higher block sizes can be configured. A quick search
for "scatter/gather" in the archives of this list reveal many accounts of
this, not least a very detailed and accurate account by Tom Peacock. Read
it.

Yura Pismerov then wrote:
> Isn't it recommended to keep the default block size 0 (variable length)
> in Linux ?
Yes it is, but this is because setting the block size to 0 allows NetWorker
to use the values configured. If your don't do this, Linux will only allow
you to used a fixed block size, and if this is only 512 bytes it will
seriously restrict the system's performance.

Kit Cunningham then suggested:
> My experince is windows2k networker uses the variable setting for writing
> only and the registry change for writing/reading. I do not know why.
This is wrong. The block size is set by setting the relevant environment
variable. However this won't work if the scatter/gather buffer settings are
insufficient, so it will just set it to a lower value - giving the effects
described. YOU NEED TO DO BOTH!

and he continued:
> I suggest ( if not already suggested by someone) you use st.conf entries
> to set your blocksize.
This is a pointless blind alley, because st.conf doesn't set the block
size. The application (NetWorker) does this, provided that you have set
other factors in your operating system to allow this (e.g. scatter/gather
buffers, stinit.def and so on).

Claudio Ruzza's posting is spot on, and is a refreshing change. However it
is not quite as complete an account as the aforementioned post by Tom
Peacock.

Teresa, I hope you have all the answers you need now. It must be very
difficult filtering out the truth from all this confusion.

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