what is media sharing and media multiplexing

[ksgnathan]

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

[Nicolai · Accepted solution]

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]

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