cancel
Showing results for 
Search instead for 
Did you mean: 

VDump problems

Mikel_7
Occasional Advisor

VDump problems

Hi all,
I have to dump approximately 21 GB of data in a DDS4 20/40 GB tape. I issue those commands:

vdump -C0Nuf /dev/ntape/tape0 /mountpoint1
vdump -C0Nuf /dev/ntape/tape0 /mountpoint2.
...............................................

where mountpointX are the mount points of several domains.

Normally, the use of -C key causes the dump to be compressed, so my backup have to fit in a tape that can contain 40GB compressed data. But the backup is interrupted at 91% and requires to change the tape. It seems that vdump does not compress the data.
Can anyone help to find the problem?
Thank you in advance.
Mikel.
9 REPLIES
Michael Schulte zur Sur
Honored Contributor

Re: VDump problems

Hi,

can you post the error message. Does it state that it ran out of space?

greetings,

Michael
Ralf Puchner
Honored Contributor

Re: VDump problems

it depend on the streaming of the tape (e.g. if the data will not be delivered within time) and why not using a devicename including compression? (man tape)
Help() { FirstReadManual(urgently); Go_to_it;; }
Erich Wimmer
Valued Contributor

Re: VDump problems

Mikel,
look at "man tape", here you'll find under "...tape drive supporte formats":
tape?_d0 .... 20 GB DDS-4
tape?_d1 .... 40 GB DDS-4
I believe, tape0 without a density code is the same as tape0_d0.
If you look at /etc/ddr.dbase, you'll find
tape?_d0 is uncompressed and tape?_d1 is compressed (HW-compression). Try tape0_d1, as suggested by Ralph.

Greetings, Erich
Mohamed K Ahmed
Trusted Contributor

Re: VDump problems

The -C option does teh compression trick
the command vdump -C -f /dev/ntape/tape0c is the same as the command
vdump -f /dev/ntape/tape0_d1

The compression is not always 50%, if you loked at the stats on the tape drive while running vdump, you cann see it around 40%.
Any way, there night be some files that cannot be compressed, that's why it fills the tape.

HTH

Mohamed
Alexey Borchev
Regular Advisor

Re: VDump problems

Mohammed, how to look tape stats during vdump?
I'd like to see speed, compression ratio, etc..
The fire follows shedule...
Mohamed K Ahmed
Trusted Contributor

Re: VDump problems

Is this a new3 question?, you should post/start a new thread for a new question.

Anyways, I always look at the stats on the tape library's menu, you can see all the information about the tape drive and what is its stat while writing or reading.

Mohamed
Johan Brusche
Honored Contributor

Re: VDump problems


Using the "-C" switch, imposes a high load on the CPU to calculate the compressed data.
If you do not have much spare CPU cycles, it migth mean that the system cannot feed data fast enough to the tapedrive to keep it streaming. Data padding will we used in an attempt to keep the drive streaming, with the result that you cannot store as much data as you would expect on 1 cartdrige.

The general rule is to NOT use host base compression, the "-C" is just there for OLD equipment thta does not support hardware compression.

_Johan_

_JB_
Alexey Borchev
Regular Advisor

Re: VDump problems

Hi, Mohammed!
- Yes, it probably would be better to start a new thread...
- You are lucky! I've got an ordinary streamer and no ways of monitoring it neither with it's console, nor from Tru64.

Hi, Johan!
I agree that compressing with -C will use lots of CPU. On the other hand, vdump's compression algorithm will likely give better compression ratios. (Just for desperate situations...)
The fire follows shedule...
Johan Brusche
Honored Contributor

Re: VDump problems


Having both CPU-based and drive-based compression at the same time, most probably will result into worse total compression rate.

_Johan_

_JB_