{"id":32479,"date":"2021-05-16T20:17:46","date_gmt":"2021-05-17T00:17:46","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=32479"},"modified":"2021-06-30T16:34:26","modified_gmt":"2021-06-30T20:34:26","slug":"big-sur-really-needs-real-free-space","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2021\/05\/16\/big-sur-really-needs-real-free-space\/","title":{"rendered":"Big Sur Really Needs Real Free Space"},"content":{"rendered":"

Howard Oakley<\/a>:<\/p>\n

Many of us have just installed the largest macOS update ever released by Apple (as far as I can tell), and there’s a few Macs which haven’t fared well.<\/p><\/blockquote>\n

My iMac died while making a copy of the macOS 11.3 installer, before I actually installed it. The spinning beachball showed up, and the Mac became completely unresponsive except for being able to move the cursor. I couldn’t even ssh<\/code> in. After a few minutes, the display went black. I eventually had to power it off and start it up again, but it got stuck halfway through the boot progress bar, the one that shows the desktop picture. I assume the CPU use was high because the fan got very loud. Eventually it seemed to kernel panic and stopped. Several more attempts<\/a> at booting all failed in the same way.<\/p>\n

Safe Mode, resetting the PRAM, Apple Diagnostics—none of the normal stuff<\/a> helped. I was able to boot from macOS Recovery as well as a clone. What if this were an M1 Mac that couldn’t boot from a clone? Would I be able to easily sync my files to another Mac while booted from Recovery? Of course, git<\/code> is not available, so I couldn’t push my work in progress. It turns out that rsync<\/code> is not available in Recovery, either, although the slower scp<\/code> is. Unfortunately, despite the network activity, the Mac will fall asleep after about 15 minutes of copying, and scp<\/code>—unlike rsync<\/code>—cannot resume a copy.<\/p>\n

I wanted to see what caused the problem but couldn’t find any kernel panic log files. Could that be because the SSD was full? No, Finder showed 37 GB free. I thought I remembered having more like 100 GB free, but 37 GB should be plenty to save a log file. Since Safe Mode and a different user account didn’t work, there must be something wrong with the system, despite the Signed System Volume. However, even with the 37 GB, macOS couldn’t be installed because the installer said there wasn’t enough free space. That sounded suspicious, but I offloaded another 20 GB or so of files and tried again. It still claimed not to have enough free space. It still wouldn’t boot.<\/p>\n

Well, we already know that with APFS the free space that Finder shows is a lie. Maybe the SSD really is full, and the files that I just deleted are being retained by an APFS snapshot. Unfortunately, tmutil<\/code> is not available in Recovery. Is there another way<\/a> to see and delete snapshots? It turns out that diskutil<\/code> can also do this. You can’t read the man page because man<\/code> is not available in Recovery. But it does show some usage information if you invoke it without any arguments, or as diskutil apfs<\/code> to see the APFS commands.<\/p>\n

Here are the steps that I used:<\/p>\n

    \n
  1. Even though Disk Utility showed my SSD’s Data volume as mounted, it was not actually available to the diskutil<\/code> snapshot commands in Terminal. I had to mount it using diskutil apfs unlockVolume<\/code>.<\/li>\n
  2. To find the APFS Volume DiskIdentifier, use diskutil apfs list<\/code>.<\/li>\n
  3. To see the snapshots, use diskutil apfs listSnapshots [volume identifier]<\/code>. This will also show you the disk number.<\/li>\n
  4. To delete a snapshot, use diskutil apfs deleteSnapshot [disk number] -uuid [snapshot identifier]<\/code><\/li>\n<\/ol>\n

    After deleting a bunch of snapshots, enough real<\/em> free space was created that I was able to boot the iMac normally. Unfortunately, the problem has repeated twice with macOS 11.3 over the last few weeks. The same fix always works. Most recently, Finder was showing 250 GB of free space, yet apparently the disk was full. I’ve now set a reminder to periodically delete old Time Machine snapshots, since macOS is not doing this automatically when it runs out of space, and I can’t trust how much space is shown as available.<\/p>\n\n

    Previously:<\/p>\n