What is Yum doing after package download?

I'm using yum update to upgrade several Oracle Linux systems.  I'm
running it as follows:

 

1 - yum update -downloadonly 

2 - <take LVM snapshots>

3 - yum update

 

I do the -downloadonly to save space on the LVM snapshots, which I use
for falling back if the upgrade is unsuccessful.

 

After running step 3, "yum update", I get the following (abbreviated)
output:

 

 

Transaction Summary

========================================================================
===============================================================

Install     21 Package(s)

Update     488 Package(s)

Remove       0 Package(s)

 

Total size: 587 M

Downloading Packages:

Running rpm_check_debug

Running Transaction Test

Finished Transaction Test

Transaction Test Succeeded

Running Transaction

  Updating       : libgcc                                      [
1/1001]

...

 

My question.  The server seems to spend a good deal of time on the
"Running rpm_check_debug" step.  I can confirm that yum is doing work by
looking at the output of top, which shows significant CPU and memory
utilization by yum.  Yum has already downloaded metadata and packages at
this point (confirmed with a du -hs * under /var/yum/cache), and has not
begun the actual upgrade.  What work is it doing here?  Is it normal for
it to take some time (~20 minutes) to complete?

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