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Message-ID: <20120515210029.GA11932@fieldses.org>
Date: Tue, 15 May 2012 17:00:29 -0400
From: "J. Bruce Fields" <bfields@...ldses.org>
To: Josef Bacik <josef@...hat.com>
Cc: Andreas Dilger <adilger@...ger.ca>,
Boaz Harrosh <bharrosh@...asas.com>,
linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org,
tytso@....edu, jack@...e.cz
Subject: Re: [PATCH] ext4: fix how i_version is modified and turn it on by
default V2
On Tue, May 15, 2012 at 04:05:34PM -0400, Josef Bacik wrote:
> On Tue, May 15, 2012 at 01:55:33PM -0600, Andreas Dilger wrote:
> > It should be fairly straight forward to have a flag set in the ext4
> > superblock (s_state flag?) that indicates that the filesystem has
> > been exported via NFS. There might be other optimizations that can
> > be done based on this (e.g. avoid some of the directory cookie
> > hijinx that are only needed if NFS has exported the filesystem and
> > needs to keep persistent cookies across reboots).
> >
> > I think that the ext4_mark_inode_dirty() performance problem could
> > be at least partially fixed by deferring the copy of in-core inode
> > to on-disk inode to use a journal commit callback. This is far more
> > work than just setting a flag in the superblock, but it has the
> > potential to _improve_ performance rather than make it worse.
Could you give any more pointers for an ext4 ignoramus? (Where *is* the
journal commit code that would need the callback? And where is the copy
currently done?)
> Yeah Btrfs doesn't have this sort of problem since we delay inode
> updating sinc it is so costly, we simply let it hang around in the
> in-core inode until we feel like updating it at some point down the
> road. I'll put together a feature flag or something to make it be
> enabled for always if somebody turns it on.
Thanks for looking at this.
A feature flag would be an improvement over a mount option.
If the flag makes a noticeable difference to performance, then it makes
me nervous toggling it automatically. And what will we do if statx
starts returning i_version to userspace?
--b.
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