Gene:
My experience with 'dd' on many UNIX platforms is that it generally tries
not
to make assumptions about block size ,and unless specified, will generally
take
whatever is the device default. The value of the default will often
surprise you.
That being said, I have had my hands bitten by 'dd' enough in the past that
I ALWAYS specify a block size on any transfer using the tool. Because of
the
need for 'dd' to get down and dirty with the tape hardware and the
undeniable
fact that many UNIX implementations handle the subtleties of device I/O
differently, it is almost always a good idea to make sure your commands know
exactly what you want done. Block size is a prime example.
'dd' is one of the original, old-school UNIX commands and make a good
illustration of why UNIX is like an experienced craftsman's tool:
A consumer-grade tool is like a ShopSmith (a 1950's era hobbyist
tool),
with lots of bells and whistles, and lots of shields and guards to
keep
the casual user's fingers out of the dangerous areas and away from
sharp
edges (and keep the plaintiff's lawyers out of the vendors pockets).
A UNIX system is like a blacksmith's shop. The tools ALL have sharp
edges and are dangerous for the inexperienced user to be around. An
apprentice is expected to be supervised by a master until the master
is sure that the apprentice will not cut off something vital.
The master is expected to know what works and what does not and
where to safely make short-cuts. The master generally has several
old
scars that demonstrate gaining that knowledge.
UNIX has gotten somewhat away from its original roots, where the
standard failure response from a command was to print '?' on the
command line.
There is still room for improvement.
Donald L. (Don) Ritchey
Information Technology
Exelon Corporation
-----Original Message-----
From: Gene Heskett [mailto:gene.heskett AT verizon DOT net]
Sent: Thursday, January 06, 2005 1:18 PM
To: amanda-users AT amanda DOT org
Subject: Re: Problem with Backup after changing hardware
On Thursday 06 January 2005 12:01, Jon LaBadie wrote:
>On Thu, Jan 06, 2005 at 03:14:20AM -0500, Gene Heskett wrote:
>> .... If the scratch file is not 512 bytes, the default amanda
>> size
>
>You shouldn't spread false info :)
>
>Amanda's default is 32K.
>dd's default is 512.
Odd. That wasn't my experience here Jon.
If I did a 'dd if=/dev/nst0 of=scratch', if the tape had been set for
32768 blocksize, then thats the size of a scratch file I got, without
telling dd the bs=32768. If that tape had not been set for 32768,
then I assume the default must have been 512, and thats the size of
the scratch file I'd get.
When I was converting my tapes, I'd read out the label block, and use
the syntax to make dd pad it on out to 32768 on the write back, IIRC
thats a -s or something.
Maybe there's different logic in other platforms dd? I wouldn't know.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
99.31% setiathome rank, not too shabby for a WV hillbilly
Yahoo.com attorneys please note, additions to this message
by Gene Heskett are:
Copyright 2004 by Maurice Eugene Heskett, all rights reserved.
************************************************************************
This e-mail and any of its attachments may contain Exelon Corporation
proprietary information, which is privileged, confidential, or subject
to copyright belonging to the Exelon Corporation family of Companies.
This e-mail is intended solely for the use of the individual or entity
to which it is addressed. If you are not the intended recipient of this
e-mail, you are hereby notified that any dissemination, distribution,
copying, or action taken in relation to the contents of and attachments
to this e-mail is strictly prohibited and may be unlawful. If you have
received this e-mail in error, please notify the sender immediately and
permanently delete the original and any copy of this e-mail and any
printout. Thank You.
************************************************************************
|
