Forum Discussion

Amaan's avatar
Amaan
Level 6
13 years ago

Tape capacity

Does NetBackup knows about tape capacity? If No how does it know if the tape is full?

thanks for answers!

7.1.x and Earlier
Backup and Recovery
NetBackup
  • mph999's avatar
    mph999
    13 years ago

    Technically, NBU knows nothing about tape capaicty - the density, hcart, hcart2 etc ... have no technical meaning, they are just a label.

    It would be better to replace the densities with colours, green, red, blue - a green tape will ONLY go into a green drive and so on.  For example, I have a 4mm drive on one server, I can call that hcart3 and the tapes hcart3 , it will work fine, but the capacity of the tape remains the same.

    THE EOT marker mentioned by JH is detected by the tapedrvie firmware, not directly by NBU.

    Aside of not understanding tape capacity, NBU  does NOT actually write data to tapes directly.  It passes the data to the operating system, and requests it is written to tape using block size x.  NetBackup then relies on the operating system to complete the task.  This is the reason that in more cases than not, tape drive issues are not the fault of NetBackup.

    The data is sent from NetBackup to the operating system, a block at a time - at some point the EOT marker is detected by the tape drive - there is sufficient physical tape left to finish writing that block, but at this point, the firmware causes a 'tape full' flag to be set in the tape driver.

    NetBackup then sends the next block of data, at this time, the tape driver (via the os) refuses the data, and a 'tape full' message is passed back to NetBackup.  At this time, a new tape is loaded.

    Regards,

    Martin 

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  • J_H_Is_gone's avatar
    J_H_Is_gone
    Level 6
    13 years ago

    NB knows a tape is full when writing to the tape it find the EOT (End Of Tape) Marker - it will then tag the tape as full.

    How much data you get on a tape depends on compression ( some stuff compress real well and you can get more than you should - and some not at all so you get a lot less then you want)

  • mph999's avatar
    mph999
    Level 6
    13 years ago

    Technically, NBU knows nothing about tape capaicty - the density, hcart, hcart2 etc ... have no technical meaning, they are just a label.

    It would be better to replace the densities with colours, green, red, blue - a green tape will ONLY go into a green drive and so on.  For example, I have a 4mm drive on one server, I can call that hcart3 and the tapes hcart3 , it will work fine, but the capacity of the tape remains the same.

    THE EOT marker mentioned by JH is detected by the tapedrvie firmware, not directly by NBU.

    Aside of not understanding tape capacity, NBU  does NOT actually write data to tapes directly.  It passes the data to the operating system, and requests it is written to tape using block size x.  NetBackup then relies on the operating system to complete the task.  This is the reason that in more cases than not, tape drive issues are not the fault of NetBackup.

    The data is sent from NetBackup to the operating system, a block at a time - at some point the EOT marker is detected by the tape drive - there is sufficient physical tape left to finish writing that block, but at this point, the firmware causes a 'tape full' flag to be set in the tape driver.

    NetBackup then sends the next block of data, at this time, the tape driver (via the os) refuses the data, and a 'tape full' message is passed back to NetBackup.  At this time, a new tape is loaded.

    Regards,

    Martin