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yilkilicaslan's avatar
yilkilicaslan
Level 4
12 years ago

What is the difference between multiplexing and "Allow multiple data streams"?

Hi everybody;

Maybe it was answered before but i am really wonder about this question:

What is the difference between multiplexing and "Allow multiple data streams"?

If i want to send a ndmp volume to a storage unit whose "concurrent write drives" set to 4, will it write this volume to 4 different drives? Or to do that which setting do i need to set:multiplexing or allow multiple data streams?

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  • mph999's avatar
    mph999
    12 years ago

    "I assume that i have 2 ndmp volume for example A and B in a policy and set to media multiplexing to 2. Does it mean that data for volume A and B start to write to tape at the same time?"

    Yes, the policy would send upto 2 streams to a single drive, you would need multiple streams set also.

    "And i assume that i have a one policy and it is including one ndmp volume. What if i want to backup this volume to 2 or more drives at the same time? What shouıld i set? "

    I don't do much configuring of NDMP backups, but to multistream from one volume you would have to specify a lower level than the volume, eg. 'directory' level and specify these under new_stream in the policy file selection.

    NEW_STREAM

    aaa

    bbb

    NEW_STREAM

    ccc

    ddd

     

    Martin

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  • mph999's avatar
    mph999
    Level 6
    12 years ago

    Multiple data streams is the ability to send more than one stream of data from a policy.

    The data on tape would look a bit like this (c1 = client 1 etc ) / EH = empty header (end of data)

    (media header)(c1 header)(c1 data)(c2 header)(c2 data)(c3 header)(c3 data)(EH)

    (It is possible that data streams would goto separate tapes, but for showing the difference between mpx and multi streams, it is better to just consider one tape drive).

    Multiple streams would send all at once (or however many you set)

    If MPX is enabled, then the multiple streams are sent from the policy at the same time, but these multiple streams can be combined to go to a single tape.

    So the data on the tape would look a bit like this (c1=client1 etc )

    (media header)(c1 header)(c2 header)(c3 header)(c1 data)(c2 data) (c1data)(c3 data)(c2 data)(EH)

    In effect the MPX backup 'mixes the data streams' to the same tape.

    The idea is when you have slow clients, they cannot drive the tape drive quick enough.  By sending multiple streams to the same drive, you hopefully can send enough data quick enough to keep the drive writing above it's streaming speed.

    Generally I think it is reasonable to say that when you talk of multiple streams, you think about streams of data from different clients.  It is possible to run multiple streams from a single client, but care must be take if the data is coming from the same disk as performance will drop.  If the client has multiple disks, you can group the data of each into separate streams, so multiple streams off one client, but these streams are coming from separate disks.

    Martin

  • yilkilicaslan's avatar
    yilkilicaslan
    Level 4
    12 years ago

    Martin,

    Thanks for the answer. I assume that i have 2 ndmp volume for example A and B in a policy and set to media multiplexing to 2. Does it mean that data for volume A and B start to write to tape at the same time?

     

    And i assume that i have a one policy and it is including one ndmp volume. What if i want to backup this volume to 2 or more drives at the same time? What shouıld i set? 

  • mph999's avatar
    mph999
    Level 6
    12 years ago

    "I assume that i have 2 ndmp volume for example A and B in a policy and set to media multiplexing to 2. Does it mean that data for volume A and B start to write to tape at the same time?"

    Yes, the policy would send upto 2 streams to a single drive, you would need multiple streams set also.

    "And i assume that i have a one policy and it is including one ndmp volume. What if i want to backup this volume to 2 or more drives at the same time? What shouıld i set? "

    I don't do much configuring of NDMP backups, but to multistream from one volume you would have to specify a lower level than the volume, eg. 'directory' level and specify these under new_stream in the policy file selection.

    NEW_STREAM

    aaa

    bbb

    NEW_STREAM

    ccc

    ddd

     

    Martin

  • mph999's avatar
    mph999
    Level 6
    12 years ago

    Opps ... missed a bit

    "If i want to send a ndmp volume to a storage unit whose "concurrent write drives" set to 4, will it write this volume to 4 different drives? Or to do that which setting do i need to set:multiplexing or allow multiple data streams?"

    No, the volume would be sent as one stream.

    If the backup selection is defined as a sinlgle volume, you can only ever send this as one stream.