See if a long process is stuck or is it reading or writing to disk
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Imagine you have a process runing but it seems stuck, here is how to see waht its doing

# apt-get install strace
# apt-get install sysstat

Follow a running process (That might seem stuck)
Get its pid:

# ps aux

Follow it:

# strace -p PID

EXAMPLE : i notice reads that are happening with 4096 block size (thats 4k)

on a long you can see if its reading or writing and at what blocksize

# iostat -t 5

NOTE: -t just to show you time, its optional

Take “kB_read/s” divide by “tps” (transactions per second, io per second)

Should be the same, I get 4 (meaning 4k)

Thus in my situation btrfsck –fix-crc <dev>  looks stuck but its not
Do an experiment
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copy a big file that will take some time to copy

# strace cp bigfile bigfilecopy

You will notice that its copying at 32768 chunk

doing a

# iostat -t 10

shows that its copying copying at 32k

# strace dd if=bigfile of=bigfilecopy512 bs=512

you will notice that its copying at 512 byte block size just as expected

CONCLUTION: copy is like a dd with a blocksize set to 32k