Re: Difference Between RMAN Incremental and Full Backups - Driving me Nuts!

On Thu, Jul 17, 2008 at 7:20 AM, David Barbour <david.barbour1_at_gmail.com> wrote:

> Incremental backup (Level 1): System slows to a crawl. People complain.
> Jobs take hours longer than expected (it's like they don't even start until
> the backup completes). Statspack snapshot covering period when backup is
> running shows top 5 timed events as follows:
>
> ...
> What is the difference between disk access for the two types of backups?
>

Level 0 incremental (full)

Oracle fills the buffer, sends it to tape

Level 1 incremental

Oracle scans files looking for blocks. When the buffer is full it is sent to tape

Here's what I suspect is occurring:

During the level 1 backup not a lot of changed blocks are being found, and Oracle is reading through datafiles as fast as possible. eg. io rates go up a lot.

When doing a level 0, there are many more waits when writing to tape, and RMAN is not reading disk as often.

Could be that level 1 backups are also killing the read cache on your IO system
due to RMAN scanning the datafiles with far fewer interruptions for tape writes.

Possible solutions?

*Less frequent level 1 backups.
Determine how many archive logs are being generated between Level 1 backups, perhaps you can rely more on the archive logs for recovery and reduce the frequency of Level 1 backups.

This would be at the cost of recovery time however.

*Upgrade to 10gR2 and use Block Change Tracking BCT was introduced in 10gR1, but was not fully baked until R2

-- 
Jared Still
Certifiable Oracle DBA and Part Time Perl Evangelist

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