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DPM and tape verify RRS feed

  • Question

  • I tried to google about this question but I didnt find an answer, the point is : I have a MSL4048 HP library of LTO4 tapes, and as long as LTO tape drives have buit-in mechanism for error detection and correction do we need to use the "check backup for data integrity" option when creating a protection group? What is the real meaning of this option ?

    I read on another thread that "Verify calculates the checksum of the data written on tape and compares it with checksum which is also written on the tape." But this applies to LTO :

    "LTO Error detection and correction 

    The tape drives use a strong error correction algorithm that makes data recovery possible when lost data is within one track. Also, when data is written to the tape it is verified by reading it back using the read heads that are positioned just 'behind' the write heads. This allows the drive to write a second copy of any data that fails the verify without the help of the host system."

    So, do we really need to enable verify ?

    As a matter of fact I use this option on my protection groups that use  tapes but now Im considering remove this option because it adds long delays on tape backups - as long as it is not really necessary.

    Thanks for any help

    Friday, June 7, 2013 2:19 PM

Answers

  • Hi,

    Best practice would still be to "verify" your backups.  It's true that the LTO technology has gotten better and tape errors are far and few between, but the verify option is an added insurance policy to ensure the data can be reliably restored.    You may want to do periodic verifies if the backup window is small.


    Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread. Regards, Mike J. [MSFT] This posting is provided "AS IS" with no warranties, and confers no rights.

    Friday, June 7, 2013 4:18 PM
    Moderator
  • Hi,

    I'm not certain. The scenario would be that a bits changed after being written and verified at time of backup.  A verify would catch the bit flips after a rewind and re-read.  How probable that is undeterminable.   If you are asking if anybody on earth has ever experiences a bad restore from a tape that just completed a successful backup on LTO tape, I would have to pass on answering that.


    Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread. Regards, Mike J. [MSFT] This posting is provided "AS IS" with no warranties, and confers no rights.

    • Marked as answer by SMFV Friday, June 7, 2013 6:10 PM
    Friday, June 7, 2013 5:20 PM
    Moderator

All replies

  • Hi,

    Best practice would still be to "verify" your backups.  It's true that the LTO technology has gotten better and tape errors are far and few between, but the verify option is an added insurance policy to ensure the data can be reliably restored.    You may want to do periodic verifies if the backup window is small.


    Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread. Regards, Mike J. [MSFT] This posting is provided "AS IS" with no warranties, and confers no rights.

    Friday, June 7, 2013 4:18 PM
    Moderator
  • Hi

    Thanks for the help ! But do you agree that using  this option sounds like doing twice the same insurance ? You pay twice (you reduce life span of tape - one pass for backup and another for verify) and get the same protection of one pass.

    The point is : is there any possibility that the DPM verify operation will return some error after the LTO buit-in auto-correct worked on a tape?

    If there is no possibility, verify is not necessary and tape life span is doubled !

    Friday, June 7, 2013 4:52 PM
  • Hi,

    I'm not certain. The scenario would be that a bits changed after being written and verified at time of backup.  A verify would catch the bit flips after a rewind and re-read.  How probable that is undeterminable.   If you are asking if anybody on earth has ever experiences a bad restore from a tape that just completed a successful backup on LTO tape, I would have to pass on answering that.


    Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread. Regards, Mike J. [MSFT] This posting is provided "AS IS" with no warranties, and confers no rights.

    • Marked as answer by SMFV Friday, June 7, 2013 6:10 PM
    Friday, June 7, 2013 5:20 PM
    Moderator