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Date: Wed, 12 Dec 2007 11:37:40 +1100
From: Timothy Shimmin <tes@sgi.com>
MIME-Version: 1.0
Subject: Re: mkfs.xfs ... lazy-count=1 ... not mountable by older kernels?
References: <475EE83A.3060607@ca.sophos.com> <20071211221554.GC4612@sgi.com>
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To: David Sparks <dave@ca.sophos.com>
Cc: xfs@oss.sgi.com

David Chinner wrote:
> On Tue, Dec 11, 2007 at 11:42:50AM -0800, David Sparks wrote:
>> Hi all,
>>
>> Is it expected that filesystems made with lazy-count=1 are not mountable by
>> older kernels?
> 
> That is expected. lazy-count is a mkfs option because it changes the on-disk
> format slightly, and older kernels do not understand that format. Hence
> mkfs sets a superblock feature bit to prevent the filesystem from being
> mounted on kernels that don't understand the slightly different disk format.
> 
> Cheers,
> 
> Dave.

And there will be a message in the logs but it probably isn't
overly obvious what it is talking about.

Looking at code, I presume it will come from:
         if (!XFS_SB_GOOD_VERSION(sbp)) {
                 xfs_fs_mount_cmn_err(flags, "bad version");
                 return XFS_ERROR(EWRONGFS);
         }
so there will be a msg about a "bad version".

--Tim