Reading stale file data with debugfs cat

It is due to caching. You have at least two options.

1. Use the -D flag:

    -D Causes debugfs to open the device using Direct I/O, bypassing the buf‐ fer cache. Note that some Linux devices, notably device mapper as of this writing, do not support Direct I/O. 

2. Drop buffer cache:

    echo 1 | sudo tee /proc/sys/vm/drop_caches 

See for example:

• How do you empty the buffers and cache on a Linux system?
• https://www.kernel.org/doc/Documentation/sysctl/vm.txt
• Documentation for /proc/sys/vm/ HTML (I like the plain text variant better, but hey ;)

---

If you do not pass the -D flag you can still see some action by piping the result to for example xxd:

sudo debugfs -R "cat <$(stat --printf %i data.txt)>" /dev/sda3 | xxd -a -c 32 

You will see that the cat'ed file is filled with zero bytes, and sometimes data (if enough has been written).

For example, after echo A >data.txt

00000000: 410a A. 

Then after for i in {1..7}; do echo A >>data.txt; done:

00000000: 410a 0000 0000 0000 0000 0000 0000 0000 A............... 

---

You can also monitor by using something like this:

Usage: sudo ./script file_to_monitor

It launches watch with an awk script that prints statistics for the device from /sys/block in addition to print the result of cat <inode> for the file.

#!/bin/sh if [ "$1" = "-h" ]; then printf '%s FILE\n' "$0" exit 1 fi file="$1" inode=$(stat --printf %i "$file") dev_path="$(df -P -- "$file" | awk 'END{print $1}')" dev_name="$(lsblk -no NAME "$dev_path")" dev_core="$(lsblk -no PKNAME "$dev_path")" if [ "$dev_core" = "loop" ]; then fn_stat=/sys/block/$dev_name/stat else fn_stat=/sys/block/$dev_core/$dev_name/stat fi printf 'File : %s\n' "$file" printf 'Inode: %s\n' "$inode" printf 'Stat : %s\n' "$fn_stat" printf 'Dev : %s\n' "$dev_path" printf "Continue? [yN] " >&2 read -r ans if ! [ "$ans" = "y" ] && ! [ "$ans" = "Y" ]; then exit fi watch -n 0.2 'awk \ -v inode="'$inode'" \ -v dev_path="'$dev_path'" \ "{ rs = \$3 * 512 rsk = rs / 1024 rsm = rsk / 1024 ws = \$7 * 512 wsk = ws / 1024 wsm = wsk / 1024 printf \" 1: Reads Completed : %9d\n\", \$1 printf \" 2: Reads Merged : %9d\n\", \$2 printf \" 3: Read Sectors : %9d %6d MiB %9d KiB %d bytes\n\", \$3, rsm, rsk, rs printf \" 4: Read ms : %9d\n\", \$4 printf \" 5: Writes Completed : %9d\n\", \$5 printf \" 6: Writes Merged : %9d\n\", \$6 printf \" 7: Write Sectors : %9d %6d MiB %9d KiB %d bytes\n\", \$7, wsm, wsk, rs printf \" 8: Write ms : %9d\n\", \$8 printf \" 9: I/Os in progress : %9d\n\", \$9 printf \"10: I/O ms : %9d\n\", \$10 printf \"11: I/O ms weighted : %9d\n\", \$11 printf \"\n\nFILE <%d> %s:\n\", inode, dev_path system(\"sudo debugfs -R '\''cat <\"inode\">'\'' \"dev_path\" | xxd -a -c 32\") } "' "$fn_stat"