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ksgnathan's avatar
ksgnathan
Level 2
13 years ago

what is media sharing and media multiplexing

Hi,

Anyone can explain what is media sharing and media multiplexing with examples, can't understand clearly through admin guide.

Aslo explain features, advantages and disadvantages...

 

Regards,

Siva

 

 

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  • Nicolai's avatar
    Nicolai
    13 years ago

    Media multiplexing  is primary used on tape devices. Imagining a stack of green and yellow plates. When backup one start green plate is put on the stack, right after yellow backup start. Both backup then switches turn to put their plate on the stack.

    Multiplexing optimize throughput on the writing side, but the downside is read speed since you need to go through both color of plates to find the right plate in the stack. Multiplexing also addresses the issues that tape drives do >80MB/sec but client rarely do that speed.

    Media Sharing only make sense when you have more media servers. Default Netbackup operation each media server has it set of partial full media. When media sharing is enabled, all media servers are allowed to use available partial full medias.

  • Nicolai's avatar
    Nicolai
    Moderator
    13 years ago

    Media multiplexing  is primary used on tape devices. Imagining a stack of green and yellow plates. When backup one start green plate is put on the stack, right after yellow backup start. Both backup then switches turn to put their plate on the stack.

    Multiplexing optimize throughput on the writing side, but the downside is read speed since you need to go through both color of plates to find the right plate in the stack. Multiplexing also addresses the issues that tape drives do >80MB/sec but client rarely do that speed.

    Media Sharing only make sense when you have more media servers. Default Netbackup operation each media server has it set of partial full media. When media sharing is enabled, all media servers are allowed to use available partial full medias.

  • mph999's avatar
    mph999
    Level 6
    13 years ago

    Adding to Nicolais excellent post ...

    It seems to be gnerally accepted that a MPX value of 4 gives a good balance between backup speed and restore speed.

    If the value is too high then as Nicolai explained, the restore speed suffers - a lot.

    Additionally, if MPX is suddenly enable, you need more memory (expained below).

     

     
    The total amount of shared memory that is used is equal to:
     
    (number_data_buffers * size_data_buffers) * number_tape_drives * max_multiplexing_setting
     
    For two tape drives, each with a multiplexing setting of 4 and with 16 buffers of
    256KB, the total shared memory usage would be:  (16 * 262144) * 2 * 4 = 32768 KB (32 MB)
     
     
    So from this we can see that suddenly increasing MPX to x16 across one of more drives causes a massive increase in the amount of memory required.
     
     
    Additionally ...
     
     
    From my bptm log, we see I have x12 data buffers, each with a size of 131072 ...
     
    08:55:45.665 [18200] <2> io_init: using 131072 data buffer size
    08:55:45.665 [18200] <2> io_init: CINDEX 0, sched Kbytes for monitoring = 60000
    08:55:45.665 [18200] <2> io_init: using 12 data buffers
     
     
    ... therefore, each tape drive, or each stream to a tape drive will require 131072 x 12 = 1572864
     
     
    Now this example is actually from a MPX backup with 2 streams, so you might think that the amount of shared memory will be 1572864 x2 = 3145728 
     
    No, here is the catch ...
     
    Looking down my bptm log I find these lines ...
     
    08:55:45.748 [18200] <2> mpx_setup_shm: buf control for CINDEX 0 is ffffffff79a00000
    08:55:45.748 [18200] <2> mpx_setup_shm: shared memory address for group 0 is ffffffff76800000, size is 6291456, shmid is 117440636
    08:55:45.748 [18200] <2> mpx_setup_shm: shared memory address for CINDEX 0 is ffffffff76800000, group 0, num_active 1
     
     
    So we see the amount of memory is 117440636
     
    Now, 6291456/ 1572864 = 4
     
    So, what has happened, is even though I have one tape drive, the amount of memory NBU will allocate is the amount of memory required by x4 tape drives.
     
    Martin