about chanel in RMAN 1

[maswien]

To specify multiple channels in RMAN only for backing up to tape, right?

Thanks

 

[karluk]

Not at all. There are very valid reasons for assigning multiple channels to rman disk backups. One situation is to split the backup sets across multiple directories, which can increase throughput or simply allow one to avoid running out of space on a single mount point. The syntax is

CONFIGURE DEVICE TYPE DISK PARALLELISM 2 BACKUP TYPE TO BACKUPSET;
CONFIGURE CHANNEL 1 DEVICE TYPE DISK FORMAT '{directory 1}';
CONFIGURE CHANNEL 2 DEVICE TYPE DISK FORMAT '{directory 2}';

 

[maswien]

Thank you very much karluk!

I read following from an article:

parallelization backup arhcieved by ->

1.allocating multiple channels
2.specify many files to backup
3.specifying the fileperset option in the backup command

all the above need to be specified to archieve parallelization or just specify any of them will be ok?

Thanks again!

 

[karluk]

My personal observation is that you don't need anything more than the configure commands in my earlier post. Rman seems to be fairly good at utilizing both channels and evenly splitting the database files across different backupsets.

 

[maswien]

Is there any performance benefit to specify multiple channels? Since our database is about 1 terabit, I more care about the timing spending on backup. The database is in noarchive mode, it will be shutdown monthly for the rman script to do level0 incremental backup and shutdown weekly for the script to do level1 incremental backup.

 

[karluk]

You might get faster backups, but it all depends on your architecture and where the bottleneck is in a one channel backup. If your backup channels are on separate physical devices, you should see an improvement in write times. Disk writes of the backupsets, in turn, tend to be slower than disk reads of the database files, so there is a good chance that the overall backup time will improve.

If I were you, I would definitely test this. There are too many dependencies to guess ahead of time exactly how your system will behave. One channel and multichannel backups of even a 100 GB database should show a clear trend of whether the method will help and, if it does, how many channels are needed for maximum throughput.

 

[maswien]

Yes, better do a test.

I use 4 channels this time, but the purpose is not to multiplex the backupset but speed the backup. So I write them to one disk, the timing of backup turned out to be the same as I use one channel.

I will test it on tape, hope I can find the difference.