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

Peter Larsen peter@peterlarsen.org
Fri Dec 9 17:12:27 EST 2016

On 12/09/2016 02:04 PM, James Ewing Cottrell, III 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.

I've done that - both ways. The only exception is very very large file
systems, as the addressable space for inodes are different. And yes, try
to open your Word document you wrote in Office 2016 with your Word you
got in 1998 and see what happens. Going backwards are always harder -
otherwise you're basically stating "no new features". 

Again, I've created plenty of USBs with ext4 and put them in different
distro's USB ports and they've been plenty happy.

At least start by posting an error message so we have something other
than "doesn't work" to go on.
> In other words ext4 != ext4.
> My question is...am I going to run into this with XFS?

I know of NO format that is backwards compatible unlimited like that.
You may be able to between some versions, but not others. With NFS, you
have to specify versions to use - it's easier with protocols. When it
comes to the actual format on a block device - well if your kernel
doesn't have a struct that covers the data being presented, or
understand the extensions used, yeah you're not going to be in for a
good time.

> Jim
>> On December 9, 2016 at 1:24 PM Peter Larsen via Novalug <novalug@firemountain.net> wrote:
>> On 12/09/2016 11:53 AM, James Ewing Cottrell III via Novalug wrote:
>>> OK, obviously I've go some 'Splainin' to do...
>>> What I mean is, if I create an ext4 fs with RHEL6, I can't read it
>>> with RHEL6; nor can I read a RHEL6 ext4 fs with RHEL5. The reason is
>>> that newer systems use newer features, which are unknown to older
>>> systems.
>> James, would you like to explain a bit further? I quite often use EXT4
>> to format USB drives (who needs vfat when everything you have is Linux)
>> and I've NEVER had a problem moving between releases, Fedora/RHEL etc.
>> when it comes to the same file system. Do know, that ext4 was TP in RHEL
>> 5, not fully supported until 5.6 but beyond that you should have no
>> issues moving between RHEL5,6,7 using ext4. 
>> Also, and this may sound a bit like an off the cuff remark, but I have
>> to ask: Are you sure your RHEL7 is using ext4? The default is xfs.
>>> In other words, it's not Upward Compatible.
>> Ehhh, it is - upwards it is. Backwards can be tough if you have very
>> very VERY large file systems (like 16+ TB).  Or at least it would be
>> nice to see an example of this.
>> -- 
>> Regards
>>   Peter Larsen
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  Peter Larsen

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