Duration and impact of online resize of a large(ish) file system

Hi,

I recently did a resize of a relatively large file system (ext3 on RHEL5), as described in this ServerFault thread: http://serverfault.com/questions/410325/online-resize-of-large-ext3-file-system-is-performance-affected

When trying to plan for duration and performance impact of increasing a 4TB filesystem with 1TB, I first did a read-only fsck, which ran for 5 hours. Therefore, due to advice that resize2fs will take 1.7-2x the time of an fsck, I budgeted 8-10 hours and running the resize in a "quiet" window. However, when I finally did the resize, it completed in 20 minutes.

Does anyone have insights on why it completed so much quicker than the read-only fsck, and how to plan for similar actions in the future?

Is there or not reason to worry about an performance impact while doing online resizing?

best regards,

Martin