Message-ID: <555213E9.90701@gmail.com>
Date: Tue, 12 May 2015 10:53:29 -0400
From: Austin S Hemmelgarn <ahferroin7@gmail.com>
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To: "J. Bruce Fields" <bfields@fieldses.org>, John Stoffel <john@stoffel.org>
CC: Kevin Easton <kevin@guarana.org>, "Theodore Ts'o" <tytso@mit.edu>,
        Sage Weil <sage@newdream.net>,
        Trond Myklebust <trond.myklebust@primarydata.com>,
        Dave Chinner <david@fromorbit.com>, Zach Brown <zab@redhat.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API Mailing List <linux-api@vger.kernel.org>
Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag
References: <CAHQdGtQjMHA8rVPkggB2zMz=k3O667+APH_1EY_2FtYmHL7-hw@mail.gmail.com> <20150508221325.GM4327@dastard> <alpine.DEB.2.00.1505081517470.28239@cobra.newdream.net> <CAHQdGtTFTN2XuvmarFZ9HPQV=cuhh7FosdHSrJME_U4htr=i8w@mail.gmail.com> <20150511144719.GA14088@thunk.org> <alpine.DEB.2.00.1505110920520.28239@cobra.newdream.net> <20150511231021.GC14088@thunk.org> <20150512050821.GA9404@chicago.guarana.org> <5551E7EB.8040301@gmail.com> <21842.1555.38099.868100@quad.stoffel.home> <20150512143637.GA6370@fieldses.org>
In-Reply-To: <20150512143637.GA6370@fieldses.org>
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On 2015-05-12 10:36, J. Bruce Fields wrote:
> On Tue, May 12, 2015 at 09:54:27AM -0400, John Stoffel wrote:
>>>>>>> "Austin" =3D=3D Austin S Hemmelgarn <ahferroin7@gmail.com> writes=
:
>>
>> Austin> On 2015-05-12 01:08, Kevin Easton wrote:
>>>> On Mon, May 11, 2015 at 07:10:21PM -0400, Theodore Ts'o wrote:
>>>>> On Mon, May 11, 2015 at 09:24:09AM -0700, Sage Weil wrote:
>>>>>>> Let me re-ask the question that I asked last week (and was appare=
ntly
>>>>>>> ignored).  Why not trying to use the lazytime feature instead of
>>>>>>> pointing a head straight at the application's --- and system
>>>>>>> administrators' --- heads?
>>>>>>
>>>>>> Sorry Ted, I thought I responded already.
>>>>>>
>>>>>> The goal is to avoid inode writeout entirely when we can, and
>>>>>> as I understand it lazytime will still force writeout before the i=
node
>>>>>> is dropped from the cache.  In systems like Ceph in particular, th=
e
>>>>>> IOs can be spread across lots of files, so simply deferring writeo=
ut
>>>>>> doesn't always help.
>>>>>
>>>>> Sure, but it would reduce the writeout by orders of magnitude.  I c=
an
>>>>> understand if you want to reduce it further, but it might be good
>>>>> enough for your purposes.
>>>>>
>>>>> I considered doing the equivalent of O_NOMTIME for our purposes at
>>>>> $WORK, and our use case is actually not that different from Ceph's
>>>>> (i.e., using a local disk file system to support a cluster file
>>>>> system), and lazytime was (a) something I figured was something I
>>>>> could upstream in good conscience, and (b) was more than good enoug=
h
>>>>> for us.
>>>>
>>>> A safer alternative might be a chattr file attribute that if set, th=
e
>>>> mtime is not updated on writes, and stat() on the file always shows =
the
>>>> mtime as "right now".  At least that way, the file won't accidentall=
y
>>>> get left out of backups that rely on the mtime.
>>>>
>>>> (If the file attribute is unset, you immediately update the mtime th=
en
>>>> too, and from then on the file is back to normal).
>>>>
>>
>> Austin> I like this even better than the flag suggestion, it provides
>> Austin> better control, means that you don't need to update
>> Austin> applications to get the benefits, and prevents backup software=

>> Austin> from breaking (although backups would be bigger).
>>
>> Me too, it fails in a safer mode, where you do more work on backups
>> than strictly needed.  I'm still against this as a mount option
>> though, way way way too many bullets in the foot gun.  And as someone
>> else said, once you mount with O_NOMTIME, then unmount, then mount
>> again without O_NOMTIME, you've lost information.  Not good.
>
> That was me.  Zach also pointed out to me that'd mean figuring out wher=
e
> to store that information on-disk for every filesystem you care about.
> I like the idea of something persistent, but maybe it's more trouble
> than it's worth--I honestly don't know.
>
But if we do it as a flag controlled by the API used by chattr, it=20
becomes the responsibility of the filesystems to deal with where to=20
store the information, assuming they choose to support it; personally, I =

would be really surprised if XFS and BTRFS didn't add support for this=20
relatively soon after the API getting merged upstream, and ext4 would=20
likely follow soon afterwards.

As far as support goes, I really think this will be easier to _safely_=20
implement (mount options are just too easy to arbitrarily change without =

knowing the consequences), although I think that reporting mtime as the=20
current wall time for files under this effect is important regardless of =

what methodology get's implemented.


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