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Data storing issue on Tapes

john10
Level 6

Hi All,

We are using NetBackup 7.0.1 and Dell TL4000 Tape Library for Backup's. Here the problem is our Tapes are showing the status full only after writing around 1.45 to 1.5 TB in LTO5 Tapes(according to vendor Native 1.5/compressed mode 3TB), and also LT)4 is showing full after writing only 788GB. do i need to enable compression option in policy in NetBackup(dont think so), could any one help me on this to increase the data storing capacity on Tapes.

1 ACCEPTED SOLUTION

Accepted Solutions

mph999
Level 6
Employee Accredited

Do not enable compression, this is done by the tape drive itself.

NBU does not mark the tape full by itself, what happens is that the tape drive detects the end of the tape, and sends a signal via the tape driver to NBU.  THen NBU marks the tape full.

So, if the tape is marked full 'early' either the data you are sending to it is not compressable (not all data can be compressed) - so this would be normal and there is no issue.

The other cause, if the data is compressable and the tape is marked full early, is that there is either a fault with the hardware (tape drive) itself, or there is a firmware of driver issue.

There is no way to control when a tape is marked full from within NetBackup.

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6 REPLIES 6

mph999
Level 6
Employee Accredited

Do not enable compression, this is done by the tape drive itself.

NBU does not mark the tape full by itself, what happens is that the tape drive detects the end of the tape, and sends a signal via the tape driver to NBU.  THen NBU marks the tape full.

So, if the tape is marked full 'early' either the data you are sending to it is not compressable (not all data can be compressed) - so this would be normal and there is no issue.

The other cause, if the data is compressable and the tape is marked full early, is that there is either a fault with the hardware (tape drive) itself, or there is a firmware of driver issue.

There is no way to control when a tape is marked full from within NetBackup.

mph999
Level 6
Employee Accredited

From TN:

http://clientui-kb.symantec.com/kb/index?page=content&&id=S:TECH169477

Tapes not reaching capacity
 
 
Scenario: 300 GB of Data is written to a 400 GB capacity tape
 
NetBackup passes data to the OS, one block at a time, to be written to the tape drive. NetBackup has no understanding of tape capacity. In theory, it would keep writing to the same tape "forever".
 
When the tape physically passes the logical end-of-tape, this is detected by the tape drive firmware. The tape drive firmware then sets a 'flag' in the tape driver (this would be the st driver in the case of Solaris). There is still enough physical space on the tape for the current block to be written, so this completes successfully. NetBackup then attempts to send the next block of data (via the operating system) but now the tape driver refuses, as the 'tape full' flag is set. The st driver then passes this 'tape full' message to the operating system, which passes it to NetBackup.  Only when this has happened will Netbackup request the tape to be changed.
 
Common causes of this issue are tape drive firmware, or faulty hardware.
 
There are no settings in NetBackup that influence tape capacity.  To further investigate Tape Capacity issues, Symantec recommends contacting your hardware vendor.

Marianne
Moderator
Moderator
Partner    VIP    Accredited Certified

The amount of data that fits onto a tape depends on the type of data.

Some types of data compresses better that others, while already compressed data not compress at all.

Tape drive is a hardware feature that is enabled by default by the tape driver. 
Enabling compression in backup policies will not make a difference - it will just result in slower backups.

You can read up in NBU Admin Guide I about policy compression Attribute:

Compression
Set this attribute if you want a client to compress its backups
before sending them to the server. Note that the time to compress
can increase backup time and make it unsuitable to use for all
clients.
See “Compression attribute” on page 480.

On page 481 you will find info about data types that compress well and about data types that don't.

john10
Level 6

Hi Mph/Marianne,

 

Thanks for your suggestions, and Tape Drives are having Latest firmware release from DEll (i.e D8D5), and the Hardware is replaced recently by Dell, and for the same issue my collegue had a talk with Dell support, they are suggesting to enable the compression option in policy(i too told the Data compression should be done by Hardware(By Tape Drive itself), so i posted in forums, in this case how can i stress the Hardware vendor to get it resolved ?

Marianne
Moderator
Moderator
Partner    VIP    Accredited Certified

Before you insist that Hardware vendor resolves the problem, please verify the types of data that is being backed up as per Admin Guide.

Something that you may want to confirm/check with Hardware vendor:

A hardware expert at my previous company told us that a tape drive will switch off compression if data transfer is too slow.
Apparently the hardware will do this in an attempt to keep the drive streaming and so prevent 'shoe shining'.

Maybe Martin will know if this is still the case?

mph999
Level 6
Employee Accredited

Yes, first thing is to determine what the data is.

Eg.  txt files would compress nicely

jpg files won't ...

... so there would be no point in turning on software comprression.

So if the data is of a compressed type already (eg jpg) then it won't compress again.  I'd agree with Mariannes other point, that the drive will detect compressed data and will not compress it again.

How to get Dell to resolve, well, it depends if it really is an issue.  What makes you think you have a problem ?   Did the same backups previously fit on less tapes (fill a tape with more data) or, do you just think that they should ?

What you could go is go and get a load of compressable data, a couple of GB of txt files and back them up, that is one option.  The other is just to decide if the type of data you have is compressable.

If it turns out that it really is an issue, I guess show them this post ...  and the TN I listed, which is official documentation from Symantec that we don't have anything to do with how much data fits on a tape.

Software compression isn't really a solution, even if it worked, it should only be treated as a tmp workaround.  It is slower and puts a greater overhead on the servers and is not as efficient as hardware compression.  In fact, in 6 years, I don't think I've ever seen anyone use the compression in a policy.

It''s worth noting that just because you have the latest firmware, it doesn't mean it doesn't have an issue.