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Something weird just happened. I got a warning that my disk was running out of space. I knew that it had like 15GB free before and wondered where all the free space went. I checked Activity Monitor which showed me that kernel_task wrote around 16GB of memory to disk. I kept observing and noticed that it went up at a rate of around 1GB per minute. This went on until there were 0 Bytes left on my disk. This made my macbook basically unusable. I couldn't even open Web pages and when i tried to shut it down, it froze. I did a forced reboot and the lost memory is back now. kernel_task is not writing to disk like crazy as before either. Anyway, I'd like to know what happened there. Any clues about what could've caused this?

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    It might have actually been some other application that was using up a ton of RAM, causing kernel_task to expand the page file and fill up your disk. I've seen that happen in the past. – William T Froggard Jun 28 '15 at 22:14
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This happened to me once. I used Daisy Disk to figure out where the disk usage was coming from.

In my case, there was a problem with the coresymbolicationd cache, which was growing indefinitely, and continuing through reboots and reinstalls of the latest Combo Update.

After deleting /System/Library/Caches/com.apple.coresymbolicationd, everything returned to normal.

Next time this happens, I recommend taking a look with that utility.

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Restart the computer.

Hold Command+Option+P+R on startup and verify Macintosh HD

Do a PRAM reset,

To reset your NVRAM, you use exactly the same procedure you once used to reset PRAM: Shut down your Mac, press the power button, and as soon as you hear the startup chime, hold down Command-Option-P-R. Keep holding down those keys until you hear a second startup chime. Then let go and allow your Mac to continue starting normally. Then check the Startup Disk, Display, and Date & Time panes of System Preferences to make sure they’re set the way you want them.

If you hold down Command-Option-P-R at startup and you see nothing but a gray screen that doesn’t change for several minutes—no Apple logo, no progress bar, no second startup chime—don’t panic. (This happened to me just last week.) The most likely cause is that your Mac isn’t registering the key presses due to wonkiness with a USB device. Disconnect all USB devices (except your keyboard, if it’s a wired keyboard), hold the power button down until the Mac shuts off completely, and then press it again and immediately hold down Command-Option-P-R. If that doesn’t work and you’re using an external Bluetooth keyboard, try plugging in a USB keyboard instead. If you’re able to reset the NVRAM successfully with the wired keyboard, you can disconnect it and go back to your normal Bluetooth keyboard.

Reset the SMC, if all else fails

Before you can reset your SMC, you must shut down your Mac. After that, the procedure varies depending on the type of Mac you have.

Desktop Macs: Disconnect the power cord (either from the Mac or from the AC outlet). Wait 15 seconds and plug it back in. Then wait another 5 seconds and turn the Mac back on.

Portable Macs with non-removable batteries: Make sure the Mac is plugged in to AC power. On the built-in keyboard, press and hold the Shift, Option, and Control keys on the left side and press the power button. Release all the keys at once, and then turn the Mac on normally.

Portable Macs with removable batteries: Disconnect the AC power cord and remove the battery. Press the power button, hold it for 5 seconds, and then let go. Put the battery back in, reconnect the power cord, and turn the Mac on normally.

This could be caused by a number of things, but usually it's just a glitch in macOS.

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