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October 10th, 2017 08:00
Difference between Avamar/DD replication DDR "stream" parameters?
I'm having some difficulty understanding the difference between a couple of the various vamar/DD replication "stream" parameters.
If I recall correctly, the "max-streams" parameter is the one that determines how many clients are going to be able to run at the same time. That is, if you only wanted to replicate a single client in a replication policy, you could just choose "1". If you wanted to replicate a number of clients but limit the number of concurrent replications to "4", you would select "4". And if you had a lot of clients, and you wanted to replicate as many as possible in that replication policy, you would select "8" (because you can only select values between 1 and 8).
What I'm having trouble with is the difference between the following parameters:
"max-ddr-streams" - which I always thought was how many streams each client "instance" was going to be able to use on the Data Domain(s); the default value for this appears to be "8"
"ddr-repl-max-parallel-streams" - which seems to be the same parameter, in terms of how many DDR streams will be allowed, but seems to relate specifically to replication (hence the naming, I guess); also, the default for this value is listed as "6"
They are definitely two separate parameters, so they must relate to separate circumstances. However, I can't seem to find any documentation that clarifies the difference between them. What is also confusing me is that the first parameter does actually show up as an advanced option when configuring a replication policy, and the "help" dialog for it mentions AMS replication; however, in several KB articles about AMS replication, they make a point of saying that one has to manually add the second parameter "to allocate additional Data Domain streams" for the replication.
Can someone provide some additional clarification and insight? Thank you.
connolly
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October 10th, 2017 11:00
There is a bit more info in KB470422 (which FWIW is marked "Internal" and "Advanced") with respect to why one might change that parameter.
The main reason I was asking is that I am working with a customer who already has some replication policies configured, and some of them use the "parallel" parameter, but others don't - and I was trying to figure out why, which was not easy when you don't know what the parameter is for. The customer is setting up some additional replication policies, and I want to make sure they are using "best practices", but frankly, I can't seem to find specific instructions for what the current replication "best practices" are (and how to set them up).
FWIW, those replication policies also have the "Default Mode" selected in the "Order" section of the replication group. As someone who has been used to selecting "replicate in parallel" and "optimize VSR" on previous projects, I wasn't sure whether the AMS replication required the "default" mode to work or not. I have read that "theoretically" VSR will be "chosen" if the backup to be replicated "qualifies" for VSR to be used, but at this particular customer, for some reason almost every backup is using AMS/NCR, and a lot of them have a message in the logs that say they can't use VSR because they don't have a "base" backup to work off of.
ionthegeek
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October 10th, 2017 11:00
The ddr-repl-max-parallel-streams parameter controls how many "slices" of a large file AMS will replicate in parallel. I can't think of a good reason to change this value.
connolly
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October 10th, 2017 13:00
Ian - FWIW, could you also provide a quick review of the "max-streams" and "max-ddr-streams" parameters as well, if you have time? That way the info for all three parameters will be here in one place. Thanks.