Linux – “Converting” a filesystem from ext3 to ext4

ext2ext3ext4filesystemslinux

I've read on some Q&A on this website that one can "convert" an ext3 filesystem to ext4 without formatting, by issuing

tune2fs -O extents,uninit_bg,dir_index /dev/dev-name

And then running fsck.

I have a few questions regarding said operation, which weren't clarified in said Q&As…

Is it really converted, as in "you'd get the same results formatting (except no data loss)"? It just seems strange for me (a layman) that it doesn't require formatting.
Does it matter if you have an "old" version of tune2fs (I'd do it from a live CD – Puppy 5.4 Dec 2012 – but I guess fsck doesn't update that often)?
Is there a significant performance gain by switching from ext3 to ext4 on the boot partition? I didn't want to mess with it and just do the switch on /home, but if there's a big difference I'd do some reading and apply it to the boot partition as well.

Best Answer

Obviously tune2fs just configures the filesystem, it does not rewrite data on disk. And obviously without rewriting you do not get the same results. The ext4 features are applied to new data only. Thus if you really want all benefits then you have to backup the data, format the volume as ext4 (or make the changes with tune2fs and then delete all the data) and restore the data.

A less radical approach is to have the tune2fs approach and copy one directory (tree) at a time, as the free space on the volume allows.

Related Solutions

How are files laid out in ext2/ext3/ext4

PhotoRec scans a disk (or disk image) searching for contiguous chunks of bytes-that-look-like-known-file-formats (for example, it can recognize JFIF/EXIF (JPEG) by the segment headers). Pretty simple but limited.

The Sleuth Kit is a great tool for digging into filesystems. With a bit of care (and scripting its tools and hex-editing the disk image when it goes astray), it can be used for recovery.

For a tool that more deeply understands ext, try ext4magic? (I haven't yet had a need to use this yet.)

Documentation/filesystems/ext2.txt in the kernel sources has a high-level overview of the general structure. The Ext4 Wiki has good information, including Ext4 Disk Layout containing more details (largely applying to ext[23] as well).

But yes, a file's data is split up into blocks. In ext2, each file is represented by an inode which contains direct blocks (pointers to data blocks), indirect blocks (which contain pointers to direct blocks), double indirect blocks, and triple indirect blocks. There are no backlinks, so to find a data block's siblings, you'll have to scan all inodes and block pointers to find its owner first.

Linux – How to Identify ext2, ext3, or ext4 Partitions

How do I tell what sort of data (what data format) is in a file?
→ Use the file utility.

Here, you want to know the format of data in a device file, so you need to pass the -s flag to tell file not just to say that it's a device file but look at the content. Sometimes you'll need the -L flag as well, if the device file name is a symbolic link. You'll see output like this:

# file -sL /dev/sd*
/dev/sda1: Linux rev 1.0 ext4 filesystem data, UUID=63fa0104-4aab-4dc8-a50d-e2c1bf0fb188 (extents) (large files) (huge files)
/dev/sdb1: Linux rev 1.0 ext2 filesystem data, UUID=b3c82023-78e1-4ad4-b6e0-62355b272166
/dev/sdb2: Linux/i386 swap file (new style), version 1 (4K pages), size 4194303 pages, no label, UUID=3f64308c-19db-4da5-a9a0-db4d7defb80f

Given this sample output, the first disk has one partition and the second disk has two partitions. /dev/sda1 is an ext4 filesystem, /dev/sdb1 is an ext2 filesystem, and /dev/sdb2 is some swap space (about 4GB).

You must run this command as root, because ordinary users may not read disk partitions directly: if needed, add sudo in front.

Best Answer

Related Solutions

How are files laid out in ext2/ext3/ext4

Linux – How to Identify ext2, ext3, or ext4 Partitions

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