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

From: Austin S Hemmelgarn
Date: Tue May 12 2015 - 10:53:42 EST

On 2015-05-12 10:36, J. Bruce Fields wrote:
On Tue, May 12, 2015 at 09:54:27AM -0400, John Stoffel wrote:
"Austin" == Austin S Hemmelgarn <ahferroin7@xxxxxxxxx> 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 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.

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).

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 where
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 becomes the responsibility of the filesystems to deal with where to 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 relatively soon after the API getting merged upstream, and ext4 would likely follow soon afterwards.

As far as support goes, I really think this will be easier to _safely_ implement (mount options are just too easy to arbitrarily change without knowing the consequences), although I think that reporting mtime as the current wall time for files under this effect is important regardless of what methodology get's implemented.

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