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Resizing COW files - a demonstration

Let's start. Among the prerequisites, the kernel must contain my COW-resize patches, as well as the ext2-resize ones. The file system must thus obviously be ext2 for now, even though tools like GNU parted might relax this restriction.

When booting for the first time, without a parameter, the COW file is created. 
Creating "slackcow" as COW file for "root_fs_slack8.1"

An empty COW file with the same size as the backing file has been created, and it is a sparse file, so it doesn't waste any space. 
*** Resizing cow file: 210819584 -> 314572800
### got size: 314572800 (move 210819584 bytes, move them 103778816 bytes)

At the second boot, the COW file is enlarged afterwards because of the size parameter which is larger than the current file size. Of course, the debug output should not appear in the final version. 
josef@noosphere /src/Linux/images/UML $ ls -las root_fs_slack8.1 slackcow
205824 -rw-------    1 josef    josef    210763776 Apr 27 10:36 root_fs_slack8.1
   624 -rw-r--r--    1 josef    josef    210819584 Apr 27 11:30 slackcow

Now let's resize the filesystem so it covers the whole virtual partition. We login and check for the storage usage first. 
Welcome to Linux 2.4.20-4um (tty0)

darkstar login: root
Linux 2.4.20-4um.
Last login: Fri Apr  4 19:54:44 +0000 2003 on tty0.
You have mail.
root@darkstar:~# df
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/ubd/0              199309     43862    145156  24% /

This is the old size. We do however want the new one. Thus we use ext2online to enlarge the filesystem to the maximum possible size. 
root@darkstar:~# ext2online / 262144
ext2online v1.1.17 - 2001/03/18 for EXT2FS 0.5b

ext2online: resizing to 262144 blocks

OK, and what's the size now? 
root@darkstar:~# df
Filesystem           1k-blocks      Used Available Use% Mounted on
/dev/ubd/0              254127     43868    197156  19% /

If we take a look at the files again, we see that the COW file is larger than the backing file, but also includes the backing data, so this is partially a waste of space. This might be improved in the future. 
josef@noosphere /src/Linux/images/UML $ ls -las root_fs_slack8.1 slackcow
205824 -rw-------    1 josef    josef    210763776 Apr 27 10:36 root_fs_slack8.1
206220 -rw-r--r--    1 josef    josef    314654208 Apr 27 16:54 slackcow

That's it. The new luxury can now be enjoyed.

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Josef Spillner (mail)
Created: 26.04.2003