Change GPT partition file system type with parted

Question

When I created my GPT partitions using parted, I was using ext2 file system type perviously:

$ parted /dev/sdc print | grep ^12 
12      961GB   1118GB  157GB   ext2               msftdata

Now I want to change it to ext4 instead, so I use mkfs.ext4 /dev/sdc12 to reformat the partition, but when I do a parted /dev/sdc print as above, it is still showing ext2 as the file system type.

I did some reading and seems that the only way for parted to touch the file system type is the mkpartfs command, Ref: http://www.thegeekstuff.com/2011/09/parted-command-examples/. Is that so? I really don't like to use the mkpartfs command to change my file system type because I don't want to recreate my partition again. That will mess up with my partition numbering (sdc12 will be named as sdc19, for e.g.). Is there any better ways?

EDIT:

Bumped into the same problem again, and found the answer here, again, to my very own question. So, logging why I thought it was necessary to change the partition file system type:

% mount $bootdev
mount: wrong fs type, bad option, bad superblock on /dev/sdb2,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

% fsck  $bootdev
fsck from util-linux 2.20.1
e2fsck 1.42.8 (20-Jun-2013)
GptLive: clean, 21/1024000 files, 74309/4096000 blocks

% mount $bootdev
mount: wrong fs type, bad option, bad superblock on /dev/sdb2,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

I.e., I newly created a /dev/sdb2 entry with type auto in the /etc/fstab. However, mount just can't mount it. However:

% mount $bootdev /mnt/GptLive

% mount | grep $bootdev
/dev/sdb2 on /mnt/GptLive type ext2 (rw)

I.e., when mount $dev doesn't work, mount $dev /mnt/place worked just fine.

I don't know why and can't explain the fix, but just logging the fact that this is why I was looking for the solution, and how I "fixed" it.

Thanks

Solution

The GParted and parted programs give only very limited control over the partition table type code information. At best, some type codes get translated into "flags." That's what the msftdata flag is -- it denotes a Microsoft Basic Data partition (GUID EBD0A0A2-B9E5-4433-87C0-68B6B72699C7; see the Wikipedia list of GPT type codes). You can change some type codes by adjusting flags, but Linux ignores these codes, except for certain limited situations, so on a Linux-only system, you shouldn't need to muck with them. On a dual-boot system, though, you should not set a Linux partition with the Microsoft Basic Data type code; see my page on this topic for details of why this is a bad idea.

The identification of the filesystem as being ext2 in parted is done by the program examining the data within the partition. If you've created a fresh ext4 filesystem in the partition, then parted should be identifying it correctly; but there may be a bug in parted's identification. If so, I wouldn't worry about it. You might want to verify that the partition really does hold an ext4 filesystem by using blkid (as in sudo blkid /dev/sdc12) or by examining the technical details with dumpe2fs -- but the latter requires a fair amount of expertise to interpret. In fact, ext4fs is basically ext2fs with extra features, so telling the two apart can be a bit tricky.

Comments

[xpt]

That's more than comprehensive, thanks a million. Yes, in my system, Ubuntu 13.10 Saucy, parted was not able to identifying it correctly, but as pointed out, blkid can.