Message-ID: <5551E7EB.8040301@gmail.com>
Date: Tue, 12 May 2015 07:45:47 -0400
From: Austin S Hemmelgarn <ahferroin7@gmail.com>
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To: Kevin Easton <kevin@guarana.org>, "Theodore Ts'o" <tytso@mit.edu>
CC: 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: <20150507002617.GJ4327@dastard> <20150507172053.GA659@lenny.home.zabbo.net> <alpine.DEB.2.00.1505071752520.28239@cobra.newdream.net> <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>
In-Reply-To: <20150512050821.GA9404@chicago.guarana.org>
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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 apparentl=
y
>>>> 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 inod=
e
>>> is dropped from the cache.  In systems like Ceph in particular, the
>>> IOs can be spread across lots of files, so simply deferring writeout
>>> doesn't always help.
>>
>> Sure, but it would reduce the writeout by orders of magnitude.  I can
>> 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 enough
>> for us.
>
> A safer alternative might be a chattr file attribute that if set, the
> 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 accidentally
> get left out of backups that rely on the mtime.
>
> (If the file attribute is unset, you immediately update the mtime then
> too, and from then on the file is back to normal).
>
I like this even better than the flag suggestion, it provides better=20
control, means that you don't need to update applications to get the=20
benefits, and prevents backup software from breaking (although backups=20
would be bigger).


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