Differences between multiplexing and multiple date stream methods

[deadwing]

Hello,
Would anyone explain to me please the differences of the above? I've tried looking some information about it over the net and I understood that multiplexing is actually when several clients send data to one tape simoultaneously.
Ok, so that is clear, but when it is exactly efficient?
I'd like to know more about both of the methods.
Especially the advantages and disadvantages.

Help, I'm a confused girl

 

[cdpwin12]

Multiple data stream is sending multiple streams from a single client. Say you have a C:\, D:\, and E:\ drive on a server it will create a backup job for each drive. If one stream fails the other streams will continue. This can speed up backups if done correctly.

Multiplexing is configured in two locations, the schedules within the policy and the storage unit. It determines the number of multiple backup jobs that can go to a single drive in a storage unit. The storage unit overrides the policy. So if the storage unit is set to 6 and the policy schedule is set to 9 only 6 backup jobs will kick off for that policy schedule, the remaining jobs will be queued. As jobs finish the queued jobs will kick off, however only up to the least amount of multiplexing set on the SU and schedule, so in my example up to 6 jobs. This is a good way to minimize tape usuage and speed up backups. However it can slow down restores so be careful when setting this number.

 

[moudt]

use multiplexing when you have clients not able to get a drive in streaming mode. Say, you have a ultrium lto2, this drive will only start streaming at a rate of appr. 30MB/sec, if one client only is not able to put this amount of data on the wire, the drive starts shoeshining, causing even worse performance, and will wear out the drive heads. If you have a few clients each able to supply , say 8mb/sec, create a policy for those clients, and set mpx'ing to 4. This way all 4 streams will put the drive in streaming mode, giving you much better performance. Keep in mind though the previous comment on restore performance.

 

[Stumpr]

You can use both multistreaming and multiplexing at the same time.

Caution with multistreaming and multiplexing be aware of the physical drive. If you have a physical drive that is split into logical drives, and you multistream and multiplex the logical drives then you will cause the physical drive to thrash between the logical drives. OK that explaination doesn't sound to good. let me try again...
Physical drive1 contains
C:\
D:\

If you multistream and multiplex C:\ and D:\ then the physical drive1 will thrash the I/O as it tries to provide both C:\ and D:\ data for the backup image at the same time.

Bob Stump
Just because the VERITAS documentation states something as certain, that does not make it a fact and that is truth.

 

[deadwing]

I think I got the explaination, but I'm not sure.
I understood that multiplexing is when you backup several clients to one single drive in SU, right?
and to cdpwin12: in the case you've described, so only 6 jobs will be backed up?

About multi data stream, I still don't get it...
Does it mean that despite C:\, D:\ and F:\ sit on the same physical drive, there will be specific job for each one?
Besides that, where do exactly I set multistreaming?

Thanks in advance

 

[cdpwin12]

You are correct about both. A single server that has a C:\, D:\, and F:\ drive and has multiple data streams enabled will have a job kick off for each drive. So in the job monitor you will see three backups for that single client but the file list will be different in the job details.

Multi stream is set on the attributes page of the policy, all you do is check the box.

 

[deadwing]

Ok, I see now.
Can you exactly point me directly to that option?

 

[Vela]

Policies>select the policy>double-click attributes>bottom right side check box.