linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Jan Kara <jack@suse.cz>
To: NeilBrown <neil@brown.name>
Cc: Jan Kara <jack@suse.cz>, "J. Bruce Fields" <bfields@fieldses.org>,
	Jeff Layton <jlayton@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nfs@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-btrfs@vger.kernel.org, linux-xfs@vger.kernel.org
Subject: Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization
Date: Thu, 6 Apr 2017 09:22:07 +0200	[thread overview]
Message-ID: <20170406072207.GA25500@quack2.suse.cz> (raw)
In-Reply-To: <87k26ygx0d.fsf@notabene.neil.brown.name>

On Thu 06-04-17 11:12:02, NeilBrown wrote:
> On Wed, Apr 05 2017, Jan Kara wrote:
> >> If you want to ensure read-only files can remain cached over a crash,
> >> then you would have to mark a file in some way on stable storage
> >> *before* allowing any change.
> >> e.g. you could use the lsb.  Odd i_versions might have been changed
> >> recently and crash-count*large-number needs to be added.
> >> Even i_versions have not been changed recently and nothing need be
> >> added.
> >> 
> >> If you want to change a file with an even i_version, you subtract
> >>   crash-count*large-number
> >> to the i_version, then set lsb.  This is written to stable storage before
> >> the change.
> >> 
> >> If a file has not been changed for a while, you can add
> >>   crash-count*large-number
> >> and clear lsb.
> >> 
> >> The lsb of the i_version would be for internal use only.  It would not
> >> be visible outside the filesystem.
> >> 
> >> It feels a bit clunky, but I think it would work and is the best
> >> combination of Jan's idea and your requirement.
> >> The biggest cost would be switching to 'odd' before an changes, and the
> >> unknown is when does it make sense to switch to 'even'.
> >
> > Well, there is also a problem that you would need to somehow remember with
> > which 'crash count' the i_version has been previously reported as that is
> > not stored on disk with my scheme. So I don't think we can easily use your
> > scheme.
> 
> I don't think there is a problem here.... maybe I didn't explain
> properly or something.
> 
> I'm assuming there is a crash-count that is stored once per filesystem.
> This might be a disk-format change, or maybe the "Last checked" time
> could be used with ext4 (that is a bit horrible though).
> 
> Every on-disk i_version has a flag to choose between:
>   - use this number as it is, but update it on-disk before any change
>   - add multiple of current crash-count to this number before use.
>       If you crash during an update, the i_version is thus automatically
>       increased.
> 
> To change from the first option to the second option you subtract the
> multiple of the current crash-count (which might make the stored
> i_version negative), and flip the bit.
> To change from the second option to the first, you add the multiple
> of the current crash-count, and flip the bit.
> In each case, the externally visible i_version does not change.
> Nothing needs to be stored except the per-inode i_version and the per-fs
> crash_count. 

Right, I didn't realize you would subtract crash counter when flipping the
bit and then add it back when flipping again. That would work.

> > So the options we have are:
> >
> > 1) Keep i_version as is, make clients also check for i_ctime.
> >    Pro: No on-disk format changes.
> >    Cons: After a crash, i_version can go backwards (but when file changes
> >    i_version, i_ctime pair should be still different) or not, data can be
> >    old or not.
> 
> I like to think of this approach as using the i_version as an extension
> to the i_ctime.
> i_ctime doesn't necessarily change on every file modification, either
> because it is not a modification that is meant to change i_ctime, or
> because i_ctime doesn't have the resolution to show a very small change
> in time, or because the clock that is used to update i_ctime doesn't
> have much resolution.
> So when a change happens, if the stored c_time changes, set i_version to
> zero, otherwise increment i_version.
> Then the externally visible i-version is a combination of the stored
> c_time and the stored i_version.
> If you only used 1-second ctime resolution for versioning purposes, you
> could provide a 64bit i_version as 34 bits of ctime and 30 bits of
> changes-in-one-second.
> It is important that the resolution of ctime used is less that the
> fastest possible restart after a crash.
> 
> I don't think that i_version going backwards should be a problem, as
> long as an old version means exactly the same old data.  Presumably
> journalling would ensure that the data and ctime/version are updated
> atomically.

So as Dave and I wrote earlier in this thread, journalling does not ensure
data vs ctime/version consistency (well, except for ext4 in data=journal
mode but people rarely run that due to performance implications). So you
can get old data and new version as well as new data and old version after
a crash. The only thing filesystems guarantee is that you will not see
uninitialized blocks and that fsync makes both data & ctime/version
persistent. But as Bruce wrote for NFS open-to-close semantics this may be
actually good enough.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

  reply	other threads:[~2017-04-06  7:22 UTC|newest]

Thread overview: 87+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-21 17:03 Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 01/30] lustre: don't set f_version in ll_readdir Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 02/30] ecryptfs: remove unnecessary i_version bump Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 03/30] ceph: remove the bump of i_version Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 04/30] f2fs: don't bother setting i_version Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 05/30] hpfs: don't bother with the i_version counter Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 06/30] jfs: remove initialization of " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 07/30] nilfs2: remove inode->i_version initialization Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 08/30] orangefs: remove initialization of i_version Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 09/30] reiserfs: remove unneeded i_version bump Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 10/30] ntfs: remove i_version handling Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 11/30] fs: new API for handling i_version Jeff Layton
2017-03-03 22:36   ` J. Bruce Fields
2017-03-04  0:09     ` Jeff Layton
2017-03-03 23:55   ` NeilBrown
2017-03-04  1:58     ` Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 12/30] fat: convert to new i_version API Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 13/30] affs: " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 14/30] afs: " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 15/30] btrfs: " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 16/30] exofs: switch " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 17/30] ext2: convert " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 18/30] ext4: " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 19/30] nfs: " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 20/30] nfsd: " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 21/30] ocfs2: " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 22/30] ufs: use " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 23/30] xfs: convert to " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 24/30] IMA: switch IMA over " Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 25/30] fs: add a "force" parameter to inode_inc_iversion Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 26/30] fs: only set S_VERSION when updating times if it has been queried Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 27/30] xfs: avoid setting XFS_ILOG_CORE if i_version doesn't need incrementing Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 28/30] btrfs: only dirty the inode in btrfs_update_time if something was changed Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 29/30] fs: track whether the i_version has been queried with an i_state flag Jeff Layton
2017-03-04  0:03   ` NeilBrown
2017-03-04  0:43     ` Jeff Layton
2016-12-21 17:03 ` [RFC PATCH v1 30/30] fs: convert i_version counter over to an atomic64_t Jeff Layton
2016-12-22  8:38   ` Amir Goldstein
2016-12-22 13:27     ` Jeff Layton
2017-03-04  0:00   ` NeilBrown
2016-12-22  8:45 ` [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization Christoph Hellwig
2016-12-22 14:42   ` Jeff Layton
2017-03-20 21:43     ` J. Bruce Fields
2017-03-21 13:45       ` Christoph Hellwig
2017-03-21 16:30         ` J. Bruce Fields
2017-03-21 17:23           ` Jeff Layton
2017-03-21 17:37             ` J. Bruce Fields
2017-03-21 17:51               ` J. Bruce Fields
2017-03-21 18:30             ` J. Bruce Fields
2017-03-21 18:46               ` Jeff Layton
2017-03-21 19:13                 ` J. Bruce Fields
2017-03-21 21:54                   ` Jeff Layton
2017-03-29 11:15                 ` Jan Kara
2017-03-29 17:54                   ` Jeff Layton
2017-03-29 23:41                     ` Dave Chinner
2017-03-30 11:24                       ` Jeff Layton
2017-04-04 18:38                       ` J. Bruce Fields
2017-03-30  6:47                     ` Jan Kara
2017-03-30 11:11                       ` Jeff Layton
2017-03-30 16:12                         ` J. Bruce Fields
2017-03-30 18:35                           ` Jeff Layton
2017-03-30 21:11                             ` Boaz Harrosh
2017-04-04 18:31                             ` J. Bruce Fields
2017-04-05  1:43                               ` NeilBrown
2017-04-05  8:05                                 ` Jan Kara
2017-04-05 18:14                                   ` J. Bruce Fields
2017-05-11 18:59                                     ` J. Bruce Fields
2017-05-11 22:22                                       ` NeilBrown
2017-05-12 16:21                                         ` J. Bruce Fields
2017-10-30 13:21                                           ` Jeff Layton
2017-05-12  8:27                                       ` Jan Kara
2017-05-12 15:56                                         ` J. Bruce Fields
2017-05-12 11:01                                       ` Jeff Layton
2017-05-12 15:57                                         ` J. Bruce Fields
2017-04-06  1:12                                   ` NeilBrown
2017-04-06  7:22                                     ` Jan Kara [this message]
2017-04-05 17:26                                 ` J. Bruce Fields
2017-04-01 23:05                           ` Dave Chinner
2017-04-03 14:00                             ` Jan Kara
2017-04-04 12:34                               ` Dave Chinner
2017-04-04 17:53                                 ` J. Bruce Fields
2017-04-05  1:26                                 ` NeilBrown
2017-03-21 21:45             ` Dave Chinner
2017-03-22 19:53               ` Jeff Layton
2017-03-03 23:00 ` J. Bruce Fields
2017-03-04  0:53   ` Jeff Layton
2017-03-08 17:29     ` J. Bruce Fields

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20170406072207.GA25500@quack2.suse.cz \
    --to=jack@suse.cz \
    --cc=bfields@fieldses.org \
    --cc=hch@infradead.org \
    --cc=jlayton@redhat.com \
    --cc=linux-btrfs@vger.kernel.org \
    --cc=linux-ext4@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-nfs@vger.kernel.org \
    --cc=linux-xfs@vger.kernel.org \
    --cc=neil@brown.name \
    --subject='Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).