linux-kernel - Re: [RFC PATCH v1 00/30] fs: inode->i

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Date:   Mon, 3 Apr 2017 16:00:55 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dave Chinner <david@...morbit.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...hat.com>, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

On Sun 02-04-17 09:05:26, Dave Chinner wrote:
> On Thu, Mar 30, 2017 at 12:12:31PM -0400, J. Bruce Fields wrote:
> > On Thu, Mar 30, 2017 at 07:11:48AM -0400, Jeff Layton wrote:
> > > On Thu, 2017-03-30 at 08:47 +0200, Jan Kara wrote:
> > > > Because if above is acceptable we could make reported i_version to be a sum
> > > > of "superblock crash counter" and "inode i_version". We increment
> > > > "superblock crash counter" whenever we detect unclean filesystem shutdown.
> > > > That way after a crash we are guaranteed each inode will report new
> > > > i_version (the sum would probably have to look like "superblock crash
> > > > counter" * 65536 + "inode i_version" so that we avoid reusing possible
> > > > i_version numbers we gave away but did not write to disk but still...).
> > > > Thoughts?
> > 
> > How hard is this for filesystems to support?  Do they need an on-disk
> > format change to keep track of the crash counter?
> 
> Yes. We'll need version counter in the superblock, and we'll need to
> know what the increment semantics are. 
> 
> The big question is how do we know there was a crash? The only thing
> a journalling filesystem knows at mount time is whether it is clean
> or requires recovery. Filesystems can require recovery for many
> reasons that don't involve a crash (e.g. root fs is never unmounted
> cleanly, so always requires recovery). Further, some filesystems may
> not even know there was a crash at mount time because their
> architecture always leaves a consistent filesystem on disk (e.g. COW
> filesystems)....

What filesystems can or cannot easily do obviously differs. Ext4 has a
recovery flag set in superblock on RW mount/remount and cleared on
umount/RO remount. This flag being set on mount would imply incrementing
the crash counter. It should be pretty easy for each filesystem to
implement such flag and the counter but I agree it requires an on-disk
format change.
 
								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR