What Is CBS.log ?

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Component Based Servicing ( CBS ) are logs that contain information about installation or uninstallations during updates and patches.

I am sure that you probably inferred that without too much issue – but what exactly does that mean and how does it affect you?

If you are reading this article – you either are experiencing some issues with an update, see that the CBS folder has grown to a ridiculous size – or you are an uber nerd like us and just enjoy learning about technology.

Regardless of what brought you here, hopefully we have some details and actionable takeaways for you!

What Is Component Based Servicing and Why Is There A Log For It?

Way back when, in the XP days, when an update was processed the log data tied with the update would be located within the windowsupdate.log file and the associated KB######.log file.

With the advent of Windows 7/Vista – the original windowsupdate.log file remains but the patching piece is now located in the Component Based Servicing logs ( CBS.log ).

Component Based Servicing logs are the reason you see “Stage 1”, “Stage 2”, and “Stage 3” during the Service Pack 1 install. “Stage 2” and “Stage 3” exists for the registry keys and files that are normally locked during regular operation.

What Can I Do With This Data?

I have only used these logs when troubleshooting Windows updates – if there is interest I can create a guide on troubleshooting Windows updates with CBS.log but – primarily reviewing the logs will indicate if the errors are indicative of an OS Update and the specific package that failed.

Where Are The Logs Located?


Can It Be Deleted Safely?

For the most part – yes. The main file you want to keep is the CBS.log file – anything that has the ‘persist’ or ‘persist<date>’ can be safely deleted.

The ‘persist’ version is created when the log file reaches a ‘maximum’ size. I have found some locations that report this to be 35-40 megs, some a gig while I have personally seen them grow over 2 gig each.

The Cabinet files are also safe to delete – cabinet files are Microsoft’s lossless data compression files and embedded digital certificates which are used for maintaining the archive’s integrity.

Short answer: Keep cbs.log and purge the rest

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