rman

[clinton]

1.Can somebody explain to me (in simple plain english) the
purpose of allocating more than 1 channel when using RMAN in the example that I have below?
I have read the docs thoroughly but I don't seem to understand

2.What determines the number of channels to allocate?

EXAMPLE
----------

run {
allocate channel c1 type disk;
allocate channel c2 type disk;
allocate channel c3 type disk;
allocate channel c4 type disk;
allocate channel c5 type disk;

[BV1963]

Major goal is to speed up your backup!
Assume you have 3 tape drives that could be used for backup. Assume you have database one 3 disks as well( just to make my point). You can send each disk's backup to separate tape and make your backup 3 times faster(in theory).
Of course, you will have some overhead, so, you real results will depend on your database configuration and hardware that you have .

Did I make it "plain" enough?

[sambavan]

The reason is to increase the processing speed. i.e to invoke a parallel operation when compared to that of sequential backup. The parallel channel allocation is possible in Enterprise version only. On SE you have to do your entire backup using 1 channel. So the time to backup the database would grow high.

FYI: the rman has a bug when it comes to reusing the same channel after closing it.

 allocate channel t1
 release channel t1
 allocate channel t1   <-- point where you would hit the bug.

This is a problem in the case of SE and not in EE. Oracle does have a backport patch for it and you have to request for it. Patch# p1308398_8172_SOLARIS.zip. The workaround on EE for this problem is not to use the same channels.

Sam

[BV1963]

Is bug you are talking about Solaris specific?

[sambavan]

I encountered it on our Solaris-OracleSE and worked with oracle on this. It seems to be an issue on the RMAN libraries. The bug fix is available for solaris currently and further pestering could get the patch for the other platforums...

Sam

[clinton]

So what determines the number of channels to allocate?
Are ther any consequences for over allocation?

[marist89]

A good rule of thumb is to allocate one channel for each physical device you will be backing up to. If you have three tape drives, use three channels. If you're backing up to disk (one filesystem), one channel should be plenty.

[ratcheer]

We got better throughput with three channels per physical tape drive (three drives, nine channels). Tested one, two, three, and four per drive. Three was by far the best for our systems and databases. Of course, "your mileage may vary".

BTW, using Sun Solaris, Veritas NetBackup, RMAN, and a little three DLT drive Sun (really, ATL) autoloader. My main databases are each about 120-150 GB.