Truncate command not creating a hole

Question

I am trying to create a file with hole using the truncate command. I read up in some posts and one of the answers in this post says to use truncate command. Filesystem used is btrfs. This is the command

$: truncate -s 16K holes
$: du holes
$: 16      holes
$:  stat holes
File: holes
  Size: 16384           Blocks: 32         IO Block: 4096   regular file

As can be seen, its allocating 16 blocks...my understanding was that it will allocate 0 blocks as mentioned that answer as well. Did I make a mistake in understanding what truncate is doing?

That still does not answer what filesystem is used. You can show that by running mount |grep vda3 — Gerald Schneider, Oct 05 '23 at 06:18
@GeraldSchneider it shows a lot of output, but btrfs is probably what you are asking? — Shivanshu Arora, Oct 05 '23 at 06:39
You could just edit the output into your question, but yes, btrfs sounds reasonable. — Gerald Schneider, Oct 05 '23 at 06:39
An existing non-sparse big file can be truncated with truncate to a smaller size and it will not become sparse. The method you used creates sparseness when truncate creates a new file (or extends an existing file). Are you sure your truncate created holes from nothing? Try with a file that doesn't exist yet and confirm the issue. — Kamil Maciorowski, Oct 05 '23 at 06:44
@KamilMaciorowski I ran truncate command on a file which didn't exist..i.e. in this case holes files did not exist before.. — Shivanshu Arora, Oct 05 '23 at 06:47
Does crc32 holes print ab54d286? What if you do fallocate -d holes? Is the file sparse now? — Kamil Maciorowski, Oct 05 '23 at 07:02

score 1 · Answer 1 · answered Oct 05 '23 at 16:48

I can't reproduce your findings:

$ ls -la
total 24
drwxrwxr-x   2 tink tink  4096 2023-10-06 05:45 .
drwxr-x--- 180 tink tink 20480 2023-10-06 05:30 ..
$ truncate -s 16K holes
$ du holes
0   holes
$ stat holes
  File: holes
  Size: 16384       Blocks: 0          IO Block: 4096   regular file
Device: fd00h/64768d    Inode: 46402746    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/  tink)   Gid: ( 1000/  tink)
Access: 2023-10-06 05:46:36.012153267 +1300
Modify: 2023-10-06 05:46:36.012153267 +1300
Change: 2023-10-06 05:46:36.012153267 +1300
 Birth: -
$

Are you sure you're using btrfs (which supposedly supports sparseness, I can't confirm as I'm on ext4) and not something like vfat?

yes I am using btrfs...I get this filesystem name when I run mount | grep vda3. Is there any other way to know? — Shivanshu Arora, Oct 06 '23 at 03:04
Then I'm at a loss. Most likely unrelated, but what virtualisation platform are you running your centos under? — tink, Oct 06 '23 at 17:09

Truncate command not creating a hole

1 Answers1