linux-kernel - Re: atime and filesystems with snapshots (especially Btrfs)

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Message-ID: <CAB9VWqDGgWE8S4dS5izkzUGEWkUbFSanWCV5trm7VWB0=r-tXQ@mail.gmail.com>
Date:	Fri, 25 May 2012 17:59:45 +0200
From:	Alexander Block <ablock84@...glemail.com>
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: atime and filesystems with snapshots (especially Btrfs)

On Fri, May 25, 2012 at 5:42 PM, Josef Bacik <josef@...hat.com> wrote:
> On Fri, May 25, 2012 at 05:35:37PM +0200, Alexander Block wrote:
>> Hello,
>>
>> (this is a resend with proper CC for linux-fsdevel and linux-kernel)
>>
>> I would like to start a discussion on atime in Btrfs (and other
>> filesystems with snapshot support).
>>
>> As atime is updated on every access of a file or directory, we get
>> many changes to the trees in btrfs that as always trigger cow
>> operations. This is no problem as long as the changed tree blocks are
>> not shared by other subvolumes. Performance is also not a problem, no
>> matter if shared or not (thanks to relatime which is the default).
>> The problems start when someone starts to use snapshots. If you for
>> example snapshot your root and continue working on your root, after
>> some time big parts of the tree will be cowed and unshared. In the
>> worst case, the whole tree gets unshared and thus takes up the double
>> space. Normally, a user would expect to only use extra space for a
>> tree if he changes something.
>> A worst case scenario would be if someone took regular snapshots for
>> backup purposes and later greps the contents of all snapshots to find
>> a specific file. This would touch all inodes in all trees and thus
>> make big parts of the trees unshared.
>>
>> relatime (which is the default) reduces this problem a little bit, as
>> it by default only updates atime once a day. This means, if anyone
>> wants to test this problem, mount with relatime disabled or change the
>> system date before you try to update atime (that's the way i tested
>> it).
>>
>> As a solution, I would suggest to make noatime the default for btrfs.
>> I'm however not sure if it is allowed in linux to have different
>> default mount options for different filesystem types. I know this
>> discussion pops up every few years (last time it resulted in making
>> relatime the default). But this is a special case for btrfs. atime is
>> already bad on other filesystems, but it's much much worse in btrfs.
>>
>
> Just mount with -o noatime, there's no chance of turning something like that on
> by default since it will break some applications (notably mutt).  Thanks,
>
> Josef

I know about the discussions regarding compatibility with existing
applications. The problem here is, that it is not only a compatibility
problem. Having atime enabled by default, may give you ENOSPC
for reasons that a normal user does not understand or expect.
As a normal user, I would think: If I never change something, why
does it then take up more space just by reading it?
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