linux-ext4 - Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO

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Date:   Mon, 12 Sep 2022 15:20:46 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        NeilBrown <neilb@...e.de>, adilger.kernel@...ger.ca,
        djwong@...nel.org, david@...morbit.com, trondmy@...merspace.com,
        viro@...iv.linux.org.uk, zohar@...ux.ibm.com, xiubli@...hat.com,
        chuck.lever@...cle.com, lczerner@...hat.com, brauner@...nel.org,
        linux-man@...r.kernel.org, linux-api@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, ceph-devel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

* Jeff Layton:

> On Mon, 2022-09-12 at 14:13 +0200, Florian Weimer wrote:
>> * Jeff Layton:
>> 
>> > To do this we'd need 2 64-bit fields in the on-disk and in-memory 
>> > superblocks for ext4, xfs and btrfs. On the first mount after a crash,
>> > the filesystem would need to bump s_version_max by the significant
>> > increment (2^40 bits or whatever). On a "clean" mount, it wouldn't need
>> > to do that.
>> > 
>> > Would there be a way to ensure that the new s_version_max value has made
>> > it to disk? Bumping it by a large value and hoping for the best might be
>> > ok for most cases, but there are always outliers, so it might be
>> > worthwhile to make an i_version increment wait on that if necessary. 
>> 
>> How common are unclean shutdowns in practice?  Do ex64/XFS/btrfs keep
>> counters in the superblocks for journal replays that can be read easily?
>> 
>> Several useful i_version applications could be negatively impacted by
>> frequent i_version invalidation.
>> 
>
> One would hope "not very often", but Oopses _are_ something that happens
> occasionally, even in very stable environments, and it would be best if
> what we're building can cope with them.

I was wondering if such unclean shutdown events are associated with SSD
“unsafe shutdowns”, as identified by the SMART counter.  I think those
aren't necessarily restricted to oopses or various forms of powerless
(maybe depending on file system/devicemapper configuration)?

I admit it's possible that the file system is shut down cleanly before
the kernel requests the power-off state from the firmware, but the
underlying SSD is not.

Thanks,
Florian