AIX JFS Logging

[KPKIND]

Hi *,

It will be great if someone can clarify my confusion about JFS Logging.

My understanding regarding JFS logging is that, any changes to the filesystem structure are logged in the jfs log before writing on to the disk. Like file is created,deleted etc... From this I understand that in the event of a system crash, I will be able to recover the layout of the filesystem but not the data.

Does this mean that, if I have created a file called test, and when I am writing some data in to this file, the system has crashed. In such an event I will be able to recover the file that I have created but with no data.

Is my understanding correct, please correct me if I am wrong.

TIA
Kumar

 

[RodKnowlton]

You're mostly right.

Unless you're using asynchronous i/o or have buffered output in the application generating your hypothetical test file, the file will still have all of its data after the crash.

That's a pretty big "Unless", though. Most applications will use either aio or buffers, because synchronous writing (the system default) sacrifices speed for data consistency.

That said, the better applications that choose speed over consistency at the system i/o level will implement some sort of logging themselves so they can have their cake and eat it too.

Rod Knowlton
IBM Certified Advanced Technical Expert pSeries and AIX 5L
CompTIA Linux+
CompTIA Security+

 

[KPKIND]

Thanks Rod,

I am much clearer now.

Cheers
Kumar

 

[sbix]

Furthermore these applications do require a flush of cached data (especially for the logs)