0

The data being displayed on a loop device mount is incorrect when the data in the underlying file is changed.

Example:

$ mkdir drv
$ dd if=/dev/zero of=data.ext4 bs=1M count=10
$ mkfs.ext4 data.ext4
$ losetup /dev/loop0 data.ext4
$ mount /dev/loop0 drv
$ echo "abcdefg" > drv/test.txt
$ cat drv/test.txt
abcdefg
$ hexdump -C data.ext4 | grep abcd
00169000  61 62 63 64 65 66 67 0a  00 00 00 00 00 00 00 00  |abcdefg.........|
$ sed -i 's/abcd/zzzz/g' data.ext4
$ hexdump -C data.ext4 | grep zzzz
00169000  7a 7a 7a 7a 65 66 67 0a  00 00 00 00 00 00 00 00  |zzzzefg.........|
$ cat drv/test.txt 
abcdefg
$ hexdump -C data.ext4 | grep abcd
$

The string "abcd" is no longer in the backing data.ext4 file, yet when the test.txt file is cat, abcd is still seen. How can I sync the loop to properly display what the backing file truly has?

Note: --direct-io=on makes no difference to the above test.

Karthik Nedunchezhiyan
  • 777
Michael Mullin
  • 1
  • In once case (echo & cat), you are changing a file on a filesystem. In the other case (sed), you are writing directly to the disk device. Maybe there's a caching problem, or you aren't writing to the same place you think you are. You can try to manually flush the disk cashes (ie "sync;sync;sync") and then check it. – mikem Dec 22 '20 at 21:13
  • the usage of hexdump is to show that I am changing with sed what I expect I am changing. dropping the loop then redoing the loop will show the expected zzzzefg when test.txt is cat.

    There is absolutely some sort of caching going on, thats what I need to sync. I need the cache for /dev/loop0 to have the data from data.ext4 read into it somehow.

     – Michael Mullin Dec 22 '20 at 21:28
  • When you drop the loop (ie umount), it is flushing the cache to that device. Did you try running "sync;sync;sync" after your sed to see if the changed data shows up? – mikem Dec 22 '20 at 23:06

0 Answers0