On normal clean shutdown, sizeStorer information will be persisted on disk. Therefore, the sizeStorer information will be accurate on restart. However, we didn't do that before opening up a backup cursor, so the sizeStorer information may be completely outdated. 

Even if we persist the sizeStorer information right before opening up backup cursor, there is still a window when more writes could go to disk (in the form of journal) and the sizeStorer information is outdated. So this improvement can only make the sizeStorer information more accurate but cannot guarantee 100% accurate unless we can stop writes in that window.