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How to compress data using backup command.

 
ora_lion
Advisor

How to compress data using backup command.

Environment:

System:OPENVMS 6.2-1H3
Tape player: TLZ10
Tape:HP DDS3 12GB-24GB

Description of back-up erroneous data report:

The data volume is about 20GB, when BACKUP/image/log dka300:[000000...]*.*;* mka400:aaa.bck ve_set is

used for data back up and it reaches around 10GB, the tape is automatically sprung to hint to change

the tape. After the tape is changed, the system is down and nothing happens in typing the keyboard, the

system has to be rebooted (this would occur more if the tape is changed in a slower motion).

Is this practice for data back up applicable, is there any other way, is compression possible in

backing up 20GB into a tape, how to carry it out specifically and what requires are needed, your helps

shall be much appreciated, thank.
6 REPLIES 6
Hoff
Honored Contributor

Re: How to compress data using backup command.

There's a problem with this software or this hardware or this server, and an investigation into the trigger -- and not a reboot -- is warranted. A reboot? Seriously? (Is this a troll?) Really? A reboot? No! Figure out what's wrong and fix it.

To get the higher of the two densities specified for DDS/DAT and DLT-series drives (assuming the data itself is not already compressed) enable compaction on the BACKUP, INITIALIZE and the MOUNT. Specifically, see /MEDIA_FORMAT=COMPACTION.

There's no supported data compression on BACKUP (other than the drive-level compression mentioned above), and the unsupported compression latent in BACKUP requires a much more recent OpenVMS release.
Joseph Huber_1
Honored Contributor

Re: How to compress data using backup command.

In addition to /MEDIA=COMPACTION, also use a bigger block size on the tape like
/BLOCK=32768 or up to /BLOCK=65024.
(The default I think is 8192, using more space and slows down tape).

System hanging when asking for a continuation tape ? Maybe there is an operator request unanswered.
It depends on the MOUNT command used (MOUNT/NOASSIST), and wether there is an operator terminal enabled. To see if there is a operator request outstanding, better have a second terminal(window) open as operator (REPLY/ENABLE=TAPE/TEMP), and see if there will be a request issued.
http://www.mpp.mpg.de/~huber
Jon Pinkley
Honored Contributor

Re: How to compress data using backup command.

Also user /GROUP=0 to prevent BACKUP from writing an XOR redundancy block for every 10 data blocks. The DAT recording standard already has Reed Solomon error correction built in, and if that fails to restore the data, the drive will generate an error that backup won't be able to recover from anyway.

The XOR redundancy add 10% to the uncompressed data, but since it appears more random, it usually will compress less, so it effectively uses more than 10% of a compressed tape. The only time that won't be true is if the data already looks random, i.e. if it is already compressed, or has been encrypted before it is sent to the drive.

If you need to encrypt the data you send offsite, the compression in the drive will not help. You need to compress the data before you encrypt it.

Jon
it depends
Andy Bustamante
Honored Contributor

Re: How to compress data using backup command.

Are you assigning tape label or allowing backup to generate them? Backup will use xxxx## for continuation tapes. Assuming BACKUP for the first label, the second label will default to BACK02, then BACK03 and so on. You have the option of initializing tapes to match or using /IGNORE=LABEL and using your own labeling pattern. As Hoff points out, an operator request is probably being sent out, asking for an appropriately labelled tape, permission to initialize the second tape or ignore the second tape label.

It's possible the tape drive is having hardware issues and not identifying the second tape.

And the standard question, has this worked in the past or did your backup just start going to a second tape recently?

Andy
If you don't have time to do it right, when will you have time to do it over? Reach me at first_name + "." + last_name at sysmanager net
ora_lion
Advisor

Re: How to compress data using backup command.

thank you,i will try.
Wim Van den Wyngaert
Honored Contributor

Re: How to compress data using backup command.

Also make sure that your tape is INIT/DENS=DDS3. If it was initialized with DDS2 it will stay in DDS2 until you init it again.

If after all the advices are implemented the tape is still to small, then try a new tape. Your tape could be bad (some kind of bad blocks).

Wim
Wim