From: Carlos Maiolino <cmaiolino@redhat.com>
Subject: Re: Still about ext2/ext3 mount options while using ext4.ko driver
Date: Mon, 6 Jan 2014 16:28:47 -0200
Message-ID: <20140106182847.GA27709@orion.maiolino.org>
References: <20131209212531.GB4296@orion.maiolino.org>
 <20131210001353.GA19933@thunk.org>
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To: linux-ext4@vger.kernel.org
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Sender: linux-ext4-owner@vger.kernel.org

Hi Ted, and, happy new year for you and everybody :)


On Mon, Dec 09, 2013 at 07:13:53PM -0500, Theodore Ts'o wrote:
> On Mon, Dec 09, 2013 at 07:25:31PM -0200, Carlos Maiolino wrote:
> > 
> > But, there are still some mount options that IMHO needs a review of what should
> > and shouldn't be allowed while using ext4 to mount ex2/3 filesystems.
> > 
> > max_batch_time=5 #I'm not sure ir this has any gain for ext2 and if it might
> > 		  work well with ext3, any comments?
> > min_batch_time=5 #same as above
> > 
> > journal_ioprio=5 #doesn't make sense for ext2, but might be useful for ext3 FS?!
> > 
> > journal=11 #doesn't make sense for ext2
> > 
> > barrier=1 #doesn't make sense for ext2
> > 
> > barrier #doesn't make sense for ext2
> > 
> > nobarrier #doesn't make sense for ext2
> > 
> > commit=5  #doesn't make sense for ext2
> > 
> > abort  #call ext4_abort(), devel usage only, and it fails to put ext2 FS on RO,
> >        if this should be kept enabled, I can look why ext2 keep RW after its use
> > 
> 
> These are all journal-related options, so they simply have no effect
> for ext2 file systems.  (The abort mount option is a test facility
> which was introduced in the ext3 days to test whether or not the
> journal abort feature worked correctly.)  They don't make sense for
> ext4 file systems that don't have a journal enabled, so maybe the
> right thing to do is to either refuse the mount or print a warning
> message indicating that the mount option is going to be ignored since
> no journal is present.
> 

IMHO, refusing to mount looks a better option to me, although these options have
no effect over ext2, refusing to mount them might save us from some possible
future regressions and less options to test for each filesystem.

> > auto_da_alloc    #ext4 only
> > 
> > noauto_da_alloc  #same as above
> 
> In Ext2 and ext3 mode, nodelalloc is the default, but in theory a user
> could explicitly request delayed allocation using the delalloc mount
> option.  There's a bit of a philosophical question hiding here about
> whether you want to prohibit users who might want to use ext2/ext3 but
> still want to enable delalloc.
> 
>
#See my last comment

> > discard		 #ex2 shouldn't be allowed to use it ?!
> > 
> > nodiscard	 #same as above
> 
> Meh; if you use discard, and you crash in the middle of the unlink you
> could lose data.  But you use ext2 and you crash in the middle of
> something, you're likely to lose data anyway.  This just amplifies it
> a bit more.
> 
> 
> > block_validity   #ext4 and debugging purposes only, should be off for ext2/3 too
> > 
> > noblock_validity #same as above
> 
> I don't see any harm in allowing block_validity; it's a debugging
> feature that can be useful in ext2 and ext3 modes.
> 

> > 
> > i_version  # I believe this is only valid for Ext4 
> 
> Well, you can have a file system with a larger (> 128 bytes) inode
> with ext2 or ext3.  So in theory you could make i_version work.  I'm
> not sure it matters a whole lot in either direction.
> 
#See my last comment


The goal of my discussion regarding these options is exactly to address problems
that these philosophical questions and the non-well tested 'theories' might
trigger and reduce the amount of corner cases we need to test for regressions
and other things, once, IMHO, if we allow an option to be used, unless specified
in the Documentation as a devel-only feature (or something like that), we need
to have it tested.

>From one point, would be nice to have ext2/ext3 filesystems taking advantage of
features like delayed allocation and others features/options, but, from another,
it just means more things we need to keep testing for regressions, etc.

My question here is: are we willing to support and maintain these kind of
features for ext2/ext3 or should we just prohibit it? Maybe we should just leave
it as-is and explicitly describe in the Documentation that some features are not
well tested on ext2/ext3 and leave the decision of supporting or not for
enterprise distros.

Cheers,

-- 
Carlos