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I noticed my MBP got really slow, which must be strange with SSD and 8GB of RAM.
So I opened the Activity Monitor to discover a process called ubd keeps its CPU usage very high, about 120% most of the time:
I found a thread that discusses the issue but it is related to having hundreds of iCloud certificates in the keychain, which I don't.
How do I go on with diagnosing the issue?
By observing log messages in ~/Library/Logs/Ubiquity/<my username>/, I realized that Ubiquity (iCloud process) kept trying to create a file in a folder that doesn't exist:
I opened up the Terminal and created this directory and its parent directories up to .ubd:
cd ~/Library/Mobile\ Documents/
sudo mkdir .ubd && cd .ubd
sudo mkdir peer-E2B13E4F-56F7-C79B-1621-E30738B638FE-v23 && cd peer-E2B13E4F-56F7-C79B-1621-E30738B638FE-v23
sudo mkdir ftr
Then I changed the newly created directories' owner from root to myself:
cd ../..
sudo chown -R <my username> .ubd/
The fan turned silent instantly.
mkdir with the -p switch to create parent directories as needed on a specified path, rather than combining multiple mkdir and cd commands. :)
https://discussions.apple.com/thread/5208590?start=0&tstart=0 :
"Applications that can store documents in iCloud include Preview, TextEdit, Pages, Numbers, Keynote, GarageBand, and some third-party software such as "DayOne," "iA Writer," "MindNode," and perhaps others. If some or all of those applications are suddenly not working well, there may be a problem with iCloud synchronization.
Back up all data.
Open the iCloud preference pane in System Preferences and uncheck the box marked Documents & Data. See whether there's any improvement. If there is, continue.
Hold down the option key and select
Go ▹ Library
from the Finder menu bar. From the folder that opens, move this folder to the Desktop: Mobile Documents (or any other item with a name beginning that way) and move this folder to the Trash: Application Support/Ubiquity Re-enable Documents & Data, log out, log back in, and test. The folders should be recreated automatically. If the issue is resolved, delete the folder you moved to the Desktop.
If you continue to have the same problems, I suggest you contact iCloud Support."
-Linc Davis