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To: linux-btrfs@vger.kernel.org
From: Martin <m_btrfs@ml1.co.uk>
Subject: Re: What to do about snapshot-aware defrag
Date: Wed, 04 Jun 2014 14:15:01 +0100
Message-ID: <lmn64l$s0g$1@ger.gmane.org>
References: <5388ED6B.8000706@fb.com> <lmav1p$ih7$1@ger.gmane.org> <538C7A79.8070407@fb.com> <lmln77$ee0$1@ger.gmane.org> <m49fvjlghh2.fsf@coffee.modeemi.fi>
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On 04/06/14 10:19, Erkki Seppala wrote:
> Martin <m_btrfs@ml1.co.uk> writes:
> 
>> The *ONLY* application that I know of that uses atime is Mutt and then
>> *only* for mbox files!...
> 
> However, users, such as myself :), can be interested in when a certain
> file has been last accessed. With snapshots I can even get an idea of
> all the times the file has been accessed.
> 
>> *And go KISS and move on faster* better?
> 
> Well, it in uncertain to me if it truly is better that btrfs would after
> that point no longer truly even support atime, if using it results in
> blowing up snapshot sizes. They might at that point even consider just
> using LVM2 snapshots (shudder) ;).

Not quite... My emphasis is:


1:

Go KISS for the defrag and accept that any atime use will render the
defrag ineffective. Give a note that the noatime mount option should be
used.


2:

Consider using noatime as a /default/ being as there are no known
'must-use' use cases. Those users still wanting atime can add that as a
mount option with the note that atime use reduces the snapshot defrag
effectiveness.


(The "for/against atime" is a good subject for another thread!)


Go fast KISS!

Regards,
Martin