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142 Posts
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What actually making Media Appendable does?
Hi,
We are using LTO4 Tapes having capacity of 800/1600 GB.
Most of the times it writes beyond 850 GB. And when we make it appendable, it still writes more in the same tape.
Some times it has gone upto 3 TB on single LTO4 Tape (Which is amazing!!!)
We are wondering, from where that extra capacity it is getting.
If any one can explain it, it will be helpful for us.
Thanks,
Bebo2k
544 Posts
1
November 16th, 2012 00:00
Hi Ashish,
Tape capacities are only approximations. If hardware compression is off, Tape drive will default to its native capacity. if hardware compression is turned on, then the number of bytes written to a tape volume is counted as the number of uncompressed bytes written. The tape drive performs compression on the fly, typically at a ratio that fluctuates from 1.5:1 to 2:1. For that you will find that some tapes contain expected capacity, and some contain much higher than expected capacity and therefore some tapes marked as full sooner than others , It is all about and related to hardware compression algorithm in tape drives, and to compressibility of data being backed up.
Also please check the NetWorker 7.6.3 Admin Guide Page 681 , on why Tapes do not fill to capacity, It says the following (Check mainly the underlined):
Tapes may not always be filled to capacity. For example, a tape with an advertised capacity of 4,000 MB can be marked full by the NetWorker server after only 3,000 MB of data have been written to it.
To use the tape to its fullest capacity, select the highest density device driver for the device. Reasons that the server appears to fill tapes prematurely include:
◆ Write errors occur during a backup.
• With any tape error, the NetWorker server marks the tape as full.
• To prevent tape write errors, clean the tape drive regularly and use only data-quality tapes. If cleaning the drive does not help, ensure that:
– The device driver is properly configured.
– Any necessary switch settings on the tape drive are set to the manufacturer’s specifications.
• All cables are secure.
• Other potential SCSIproblems have been addressed.
◆ NetWorker filemarks consume space on the tape.
• The NetWorker server periodically writes filemarks to facilitate rapid recovery of data. These filemarks consume varying amounts of tape depending on the type of tape drive.
• The number of filemarks the server writes to tape depends on how many save sets are on the tape. Many small save sets require more filemarks than a few larger ones.
◆ Tape capacities vary.
Two apparently identical tapes from the same vendor can vary significantly in capacity. This can cause problems if you copyone full tape to another, especially if the destination tape holds less data than the source tape.
◆ Data compression affects the tape capacity.
• If you use compression on the tape drive, you cannot predict the effect on tape capacity. A compressing drive can provide twice the capacity of a
noncompressing drive.
• The capacity could vary depending on the type of data being backed up. For example, if a noncompressing drive writes 2 GB of data to a specific tape, the compressing drive could write 10 GB, 2 GB, 5 GB, or some other unpredictable amount of data.
◆ Length of tape. Verify tape lengths. A 120-meter DATtape holds more data than a
90-meter DATtape
So overall I would like to say again that it is related to hardware compression algorithm in tape drives, and to compressibility of data being backed up, and you can not predict the effect of the hardware compression on the tape capacity.
Hope this helps you and clear the image.
Thanks,
Ahmed Bahaa
ble1
14.3K Posts
1
November 15th, 2012 12:00
800GB is so called native capacity - without compression. All drives (IBM drives require IBM drivers most of the time) use compression so they are able to fill even more. Standard by vendors is to state native capacity/2:1 compression capacity on labels.
ashxos
142 Posts
1
November 16th, 2012 00:00
That thing is fine.
For writing so much data, from where it gets that much space?
There is a physical limitation to LTO5 Tape, i.e. Tape Length.
I have one more question.
On what basis Networker decides that the cartridge is full?
ble1
14.3K Posts
1
November 16th, 2012 13:00
Tape has so called marker called end-of-tape (EOT). So when writing data onto it tape drive head reaches this marker, it knows this tape has been filled until the very end. That's assuming there are no other out of ordinary conditions which may trigger tape to be marked full by backup application (like IO errors).
KiranBandi
7 Posts
0
November 29th, 2012 10:00
Adding some more info here....
Appendable media: A media that have space to write data to it. When you are making a media Appendable manually, you are increasing the retention period of savesets that were already written to that tape indirectly. Because a tape can't be overwritten untill all the savesets on that tape becomes recyclable. When you make a tape appendable, Networker uses that tape for backups by adding data to it but not by overwriting existing data. I hope i am making some sense here....
Compression ratio a backup job can achive highly depends upon the type of data that is being backedup. Text files, word documents etc does compress well. Tape drive would be able to write more data than the double size of tape's native capacity. But in reality. 1:1.4 can be considered as a good compression ratio.
Weird thing is, if already compressed files like .zip, some video format files are part of backup then compression works in reverse way i.e size after compression will be greater than actual size.
Thanks....