hiberfil.sys: what it is, and how to shrink or remove it

What the file is for

When a PC hibernates, Windows compresses its user and system state and writes it to disk, then reads it back on the next start. Microsoft lists three users of the hibernation file: standard hibernation, hybrid sleep and Fast Startup. That last one is why the file matters even if you never choose Hibernate: Fast Startup is the default kind of shutdown in Windows, and it saves the kernel session to this file.

Why it is so large

The size follows your RAM, not your files. Microsoft gives a default of 40% of physical memory for a full hibernation file and 20% for a reduced one, so on a PC with 32 GB of RAM that is about 12.8 GB or 6.4 GB. A custom size set with powercfg can make it larger, which is why a file close to the size of your RAM is not necessarily a fault. Installing more memory makes it bigger, and cleaning up your apps does not make it smaller.

Is it safe to delete?

Removing it through powercfg is safe in the sense that Microsoft documents it and one command brings it back, but it does cost you features. Do not try to remove the file in File Explorer: the supported route is the powercfg command.

• ✓Hibernate stops being available
• ✓Hybrid sleep stops working. Microsoft warns that you may lose data if the power fails while hybrid sleep is set to on
• ✓Fast Startup stops as well, because it needs the same file, so every Shut down becomes a full shutdown

Shrink it instead of removing it

If the only thing you use is Fast Startup, the reduced type is the better trade: it keeps Fast Startup and gives up Hibernate and hybrid sleep, at half the default size of a full file. Only Fast Startup can use a reduced file. If you do use Hibernate, keep the full type.