[Novalug] Ext4 != Ext4. Is XFS == XFS?

James Ewing Cottrell III jecottrell3@comcast.net
Sat Dec 10 00:19:40 EST 2016


Thanks for your Observation. I just installed CentOS 7.2 all in one 
partition (well, two if you count /boot/efi), booted up a CentOS 6.6 CD, 
went over to the ALT-F2 shell window, and was indeed able to mount the 
7.2 ext4 filesystem I just created.

However, All is Well only if All is Well. If your box crashes, you will 
not be able to fix the RHEL7 FS by using a RHEL6 fsck. It fails with:

# e2fsck -f /dev/sda7
sda7 has unsupported feature(s): 64bit

The reason this is Important is that Traditionally, I have always 
created an extra Rescue Linux on my disk. Back in the Grub1 days, the 
Rescue Linux boot code was installed in the MBR and do Chainload the 
Boot Code from any other Linuxi I installed on other partitions. This, 
of course meant actually running grub twice, but unless I messed with 
the Rescue Linux, I rarely lost Control.

Obviously, with Grub2, things have changed.


On 12/9/2016 3:26 PM, Jon LaBadie via Novalug wrote:
> On Fri, Dec 09, 2016 at 02:04:27PM -0500, James Ewing Cottrell, III via Novalug wrote:
>> OK, so I mean Backwards Compatible. Boot RHEL 7. Mkfs.ext4. Now try an read that with RHEL 6. Now make another ext4 and try and read it with RHEL5. Doesn't work.
>> In other words ext4 != ext4.
>>From my limited experience (not RHEL 6/7 but CentOS 6/7)
> I'm surprised to hear this.  I was running a system with
> 6 and upgraded by a fresh install of 7 on a new disk.
> While shaking out 7, I retained the ability to boot into
> the old 6 system (on a separate disk).  During this 2 wk
> period, I had no problems accessing either 6 or 7 file
> systems from the other.
> Jon

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