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Re: [PATCH] default to 64 bit inodes & add feature flag

To: Ben Myers <bpm@xxxxxxx>
Subject: Re: [PATCH] default to 64 bit inodes & add feature flag
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Thu, 08 Mar 2012 09:42:21 -0600
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>, "Josef 'Jeff' Sipek" <jeffpc@xxxxxxxxxxxxxx>
In-reply-to: <20120308153721.GS7762@xxxxxxx>
References: <4F5798F9.2050809@xxxxxxxxxx> <20120308013413.GL3592@dastard> <4F5813D8.8070305@xxxxxxxxxxx> <20120308153721.GS7762@xxxxxxx>
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On 3/8/12 9:37 AM, Ben Myers wrote:
> On Wed, Mar 07, 2012 at 08:05:12PM -0600, Eric Sandeen wrote:
>> On 3/7/12 7:34 PM, Dave Chinner wrote:
>>> On Wed, Mar 07, 2012 at 11:20:57AM -0600, Eric Sandeen wrote:
>>>> From: Dave Chinner <dchinner@xxxxxxxxxx>
>>>>
>>>> Default to allowing 64-bit inodes on the filesystem.
>>>>
>>>> Add a feature bit to the the superblock to record whether 64 bit inodes 
>>>> have
>>>> been allocated on the filesystem or not. This allows us to reject mounting 
>>>> the
>>>> filesytem with inode32 if 64 bit inodes are present.
>>>>
>>>> Once a 64 bitinode is allocated, the inode64 superblock feature bit will 
>>>> be set.
>>>> Once the superblock feature bit is set, the filesystem will default to 64 
>>>> bit
>>>> inodes regardless of whether inode64 is specified as a mount option.
>>>>
>>>> To ensure only 32 bit inodes are created, the inode32 mount option must be
>>>> used. If there are already 64 bit inodes as flagged by the superblock 
>>>> feature
>>>> bit, then the inode32 mount will be refused.
>>>>
>>>> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
>>>> ---
>>>>
>>>> Passing this along to revive the old discussion ... 
>>>
>>> I have no objections to do this. However, the kernel patch is just
>>> the tip of the iceberg when it comes to implementing this.
>>>
>>> Were there patches for userspace support of the feature bit? I don't
>>> recall if there were. I'm thinking that xfs_info needs to output
>>> whether this is set, which means the flag needs to be added to the
>>> xfs geometry ioctls in the kernel.
>>
>> Nope, you just put this patch out as a suggestion, and pointed out
>> that userspace needed updates too.
>>
>> If people are in agreement about this then we can proceed with the rest...
> 
> Please do.  I too have been burned by mounting a filesystem with big
> inos without the correct mount option.  This is a great idea.

So, after thinking about this (and talking on irc) some more, I'm
not convinced that a feature flag is the way to go.

If we set a feature flag, suddenly old filesystems with 64-bit
inodes will grow a new feature, and this will force a userspace
upgrade - but there is no real new feature.  This seems like a bad
idea.  My original patch (which Dave responded to with this one)
simply made inode64 default, with no feature flags.

Unless someone has a really compelling argument for the flag,
I'm thinking this is the wrong approach after all.

Perhaps I should resend the just-make-it-default patch.

Comments?

-Eric
 
> -Ben
> 

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