사실 저도 잘 모르는 문제라서요 

Cannot read files on system. Try changing your FS_METHOD to direct
위와 같은 에러 메시지가 나옵니다.
그리고 일부 플러그인이 업데이트 되지 않습니다.



WordPress Upgrade Constants
You should define as few of the below constants needed to correct your update issues.

The most common causes of needing to define these are:

Host running with a special installation setup involving symlinks. You may need to define the 
path-related constants (FTP_BASE, FTP_CONTENT_DIR, and FTP_PLUGIN_DIR). Often defining simply 
the base will be enough.
Certain PHP installations shipped with a PHP FTP extension which is incompatible with certain 
FTP servers. Under these rare situations, you may need to define FS_METHOD to "ftpsockets".
The following are valid constants for WordPress updates:

FS_METHOD forces the filesystem method. It should only be "direct", "ssh2", "ftpext", or 
"ftpsockets". Generally, you should only change this if you are experiencing update problems. If 
you change it and it doesn't help, change it back/remove it. Under most circumstances, setting 
it to 'ftpsockets' will work if the automatically chosen method does not. Note that your 
selection here has serious security implications. If you are not familiar with them, you should 
seek help before making a change.
(Primary Preference) "direct" forces it to use Direct File I/O requests from within PHP. It is 
the option chosen by default.
(Secondary Preference) "ssh2" is to force the usage of the SSH PHP Extension if installed
(3rd Preference) "ftpext" is to force the usage of the FTP PHP Extension for FTP Access, and 
finally
(4th Preference) "ftpsockets" utilises the PHP Sockets Class for FTP Access.