linux-kernel - Re: [PATCH RFC] vfs: add a O

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Message-ID: <20150511231021.GC14088@thunk.org>
Date:	Mon, 11 May 2015 19:10:21 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Sage Weil <sage@...dream.net>
Cc:	Trond Myklebust <trond.myklebust@...marydata.com>,
	Dave Chinner <david@...morbit.com>,
	Zach Brown <zab@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux API Mailing List <linux-api@...r.kernel.org>
Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag

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 apparently
> > 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 inode 
> 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.

Cheers,

					- Ted

P.S.  I do agree that if we do need this upstream, requiring a mount
option to enable the feature is probably a good compromise.
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