What happens if during RAR compression the laptop runs out of battery?

Question

I was compressing a large file using RAR, so it took quite a while. During the compression at some point I had to unplug the computer and let it continue with battery only. After a few hours when I looked at it again, the laptop had already shut down due to low battery after a few hours (I just didn't expect the battery to go down so fast). After I switched on the laptop again, I was able to resume with the compression at the point it had stopped before the laptop shut down. In such a case, is there a risk of corrupted files? Or does Windows shut the laptop automatically in a smooth way when it detects low battery?

Answer 1

after a few hours (I just didn't expect the battery to go down so fast).

File compression is a very CPU-intensive task – exactly the kind of thing that is battery-intensive as well. A good part of your battery life on a modern laptop comes from the CPU idling at its lowest frequencies, so if it has to work at full power then it'll drain it just the same. Honestly, "A few hours" already exceeds my expectations.

In such a case, is there a risk of corrupted files?

Files get corrupted when the system turns off when they're being written, not when they're being read. Archiving files does not involve writing into them – the archiver only reads them and writes only the .rar output file.

So at most you'll just have half a .rar left over in some directory (possibly in %TEMP%) occupying space because the archiver didn't have a chance to either finish or delete it. I think WinRAR stores them in %TEMP%, but it could also be in the destination folder.

Filesystem metadata corruption shouldn't be an issue; NTFS is a journaled filesystem and deals with interrupted updates well enough. It would be a problem for FAT though (you really shouldn't cancel those "Disk has a problem" prompts for FAT).

Or does Windows shut the laptop automatically in a smooth way when it detects low battery?

Depends on how bad the battery is. If it's still "runs fine at 1%" kind of new, then the OS will initiate a proper shutdown at somewhere around 5% (if I recall the default settings correctly). I think current Windows versions will hibernate instead of shutting down.

If the battery is old, however, it might literally no longer be able to power the laptop once it drains to a certain point, and the system will die before the OS has a chance to react (or more accurately, an old battery can suddenly shut off at like 20%, which is well above the OS's default setting for "critical battery percentage").