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Media Multiplexing

Hi Experts,

Previoulsy I had many jobs in queued state and these got failed with 196 errors. We have 8 drives and Media Multiplexing was set to 1 in full and differential backups

in policy schedule.

I changed these Media Multiplexing to 4 in full and incremental in schedule. Now backups are happening very fast.

1:- So i just need to know whether its right approch?

2:- We are using LTO-6 tapes. I need to know whether 1 drive can write 3-4 backup jobs in the same tape at the same time? I think thats what is happening now?

1 Solution

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Accepted Solution!

Yes. 1 drive can write 3-4

Yes. 1 drive can write 3-4 backup jobs in the same tape at the same time. 
Perfectly fine.
By writing 3-4 (slow) backups simultaneously to the drive (multiplexing) you keep the drive streaming with the increased throughput.

My experience over 15 years has been that MPX of 4 gives good backup as well as good restore performance.
You need to perform your own test restores from multiplexed backups to see if restore performance is acceptable.

Have you had a look at the section in Admin Guide (link in my previous post)? 
It explains nicely what happens during multiplexing and multistreaming with pictures that makes it easy to understand.

 

The same info can be found in these TNs: 
http://www.symantec.com/docs/HOWTO86804 
http://www.symantec.com/docs/HOWTO86888 

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7 Replies
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Hello Its fine, but there are

Hello

Its fine, but there are pro's and con's to using multiplexing. Your drives can write backups at X mbps and if your network (clients) can provide the data fast enough, then there shouldn't be a need for multiplexing.

It was used in the past where you had many slow clients that only sent data at 1/X of the tape drives max performance. So it made sense to add them together by multiplexing.

Note that multiplexing might iimprove backup speed by putting everything together on the tape, but restores are impacted because your image is now distributed across more of the tape than if there was no multiplexing.

If your clients had previously been running at maximum speed, all you achieved by mutliplexing is having more jobs start but running at a lower throughput. So compare a client from before multiplexing to the speed you get now.

 

And yes, that is what is happening. Here is a nice note explaining it more..

http://www.symantec.com/docs/DOC2799

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Multiplexing can be

Multiplexing can be configured up to 32, but real life experience has showed us that a value of 4 gives good backup performance as well as good restore performance.

You should perform restore tests from multiplexed backups to see the effect in your environment.
I have seen customers even using MPX of 8 and still satisfied with restore performance.
As I've said above - MPX of 4 is a good average.
Important to test restores each time you make a change.

In addition to the TN in Riaan's post, you can also read up in NetBackup Administrator's Guide, Volume I  from page 739 onwards under this topic: About multiplexing 
You will see diagrams on p. 740 and 744 that explains very well how it works.

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My personal experience has

My personal experience has been if writing directly to tape to always use multiplex. Like previously stated 4 is a good number to start with. I have gone as high as 10 in some occasions. Always do back/restore testing to make sure you are OK with the results. I am OK with slower restores if I can get faster backups. Backups are everyday....restores are "seldom".

 

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... I saw a multiplexed

... I saw a multiplexed restore take over 32 hours once ... That same data, backed up again, non-multiplexed, restored within a couple of hours. Another time, high mpx value was enabled across an entire system, it crashed as the media servers didn;t have enough memory ...
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Agree with bmitche: backups

Agree with bmitche: backups you have to do day in day out : thats where you want the performance, restores you dont do nearly so often <1% of the backup. For big data and high throughput I use low MPX or zero eg SAP and ORA dbs directly attached to tape, for bog standard clients and OS full backups I go for 3, for incrementals of same I go for 10.  But as Riaan says, if you're maxing the drive in any case you're gaining nothing but this is rare.

Jim

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2:- We are using LTO-6 tapes.

2:- We are using LTO-6 tapes. I need to know whether 1 drive can write 3-4 backup jobs in the same tape at the same time? I think thats what is happening now?

Highlighted
Accepted Solution!

Yes. 1 drive can write 3-4

Yes. 1 drive can write 3-4 backup jobs in the same tape at the same time. 
Perfectly fine.
By writing 3-4 (slow) backups simultaneously to the drive (multiplexing) you keep the drive streaming with the increased throughput.

My experience over 15 years has been that MPX of 4 gives good backup as well as good restore performance.
You need to perform your own test restores from multiplexed backups to see if restore performance is acceptable.

Have you had a look at the section in Admin Guide (link in my previous post)? 
It explains nicely what happens during multiplexing and multistreaming with pictures that makes it easy to understand.

 

The same info can be found in these TNs: 
http://www.symantec.com/docs/HOWTO86804 
http://www.symantec.com/docs/HOWTO86888 

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