Re: [PATCH RFC] vfs: add a O_NOMTIME flag

From: Theodore Ts'o
Date: Mon May 11 2015 - 19:10:30 EST

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.


- 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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