Why can reading (cat/less/vi) of a file with status D hang?

Question

linux

good afternoon
first encountered: my own file became unreadable today
under root I try cat / less / vi , hang
ps shows status D for the reading process It's
time to panic, what's wrong with the file system?
ls -l works fine, permissions are 644

Answer 1

D is the IO wait. Either the disk is all, or a bad contact of the disk with the interface in the computer (SATA / USB / etc), or the power disk is not enough, or 12309.
If the network FS, then there may be a plug or break in the communication line.