From: "Theodore Ts'o" <tytso@mit.edu>
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: dsterba@suse.cz, Liu Bo <bo.li.liu@oracle.com>,
linux-btrfs@vger.kernel.org, fdmanana@suse.com, kzak@redhat.com,
linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk,
linux-nfs@vger.kernel.org, chuck.lever@oracle.com,
mingming.cao@oracle.com
Subject: Re: i_version vs iversion (Was: Re: [RFC PATCH v2 1/2] Btrfs: add noi_version option to disable MS_I_VERSION)
Date: Thu, 25 Jun 2015 18:12:57 -0400 [thread overview]
Message-ID: <20150625221257.GD13380@thunk.org> (raw)
In-Reply-To: <20150625184644.GA12300@fieldses.org>
On Thu, Jun 25, 2015 at 02:46:44PM -0400, J. Bruce Fields wrote:
> > Does this sound reasonable?
>
> Just to make sure I understand, the logic is something like:
>
> to read the i_version:
>
> inode->i_version_seen = true;
> return inode->i_version
>
> to update the i_version:
>
> /*
> * If nobody's seen this value of i_version then we can
> * keep using it, otherwise we need a new one:
> */
> if (inode->i_version_seen)
> inode->i_version++;
> inode->i_version_seen = false;
Yep, that's what I was proposing.
> Looks OK to me. As I say I'd expect i_version_seen == true to end up
> being the common case in a lot of v4 workloads, so I'm more skeptical of
> the claim of a performance improvement in the v4 case.
Well, so long as we require i_version to be committed to disk on every
single disk write, we're going to be trading off:
* client-side performance of the advanced NFSv4 cacheing for reads
* server-side performance for writes
* data robustness in case of the server crashing and the client-side cache
getting out of sync with the server after the crash
I don't see any way around that. (So for example, with lazy mtime
updates we wouldn't be updating the inode after every single
non-allocating write; enabling i_version updates will trash that
optimization.)
I just want to reduce to a bare minimum the performance hit in the
case where NFSv4 exports are not being used (since that is true in a
very *large* number of ext4 deployments --- i.e., every single Android
handset using ext4 :-), such that we can leave i_version updates
turned on by default.
> Could maintaining the new flag be a significant drag in itself? If not,
> then I guess we're not making things any worse there, so fine.
I don't think so; it's a bit in the in-memory inode, so I don't think
that should be an issue.
- Ted
next prev parent reply other threads:[~2015-06-25 22:13 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-17 7:54 [RFC PATCH v2 1/2] Btrfs: add noi_version option to disable MS_I_VERSION Liu Bo
2015-06-17 7:54 ` [RFC PATCH v2 2/2] Btrfs: improve fsync for nocow file Liu Bo
2015-06-17 15:58 ` David Sterba
2015-06-18 3:27 ` Liu Bo
2015-06-24 18:21 ` David Sterba
2015-06-25 2:24 ` Liu Bo
2015-06-25 16:10 ` David Sterba
2015-06-17 15:33 ` [RFC PATCH v2 1/2] Btrfs: add noi_version option to disable MS_I_VERSION David Sterba
2015-06-17 15:52 ` Liu Bo
2015-06-17 17:01 ` David Sterba
2015-06-18 2:46 ` Liu Bo
2015-06-18 14:38 ` i_version vs iversion (Was: Re: [RFC PATCH v2 1/2] Btrfs: add noi_version option to disable MS_I_VERSION) David Sterba
2015-06-19 11:44 ` Karel Zak
2015-06-19 11:44 ` Karel Zak
2015-06-22 20:42 ` Dave Chinner
2015-06-22 20:42 ` Dave Chinner
2015-06-24 17:28 ` David Sterba
2015-06-23 16:32 ` Theodore Ts'o
2015-06-24 8:23 ` Liu Bo
2015-06-24 18:02 ` David Sterba
2015-06-24 23:17 ` Theodore Ts'o
2015-06-24 23:17 ` Theodore Ts'o
2015-06-24 23:59 ` Dave Chinner
2015-06-24 23:59 ` Dave Chinner
2015-06-25 18:46 ` J. Bruce Fields
2015-06-25 18:46 ` J. Bruce Fields
2015-06-25 22:12 ` Theodore Ts'o [this message]
2015-06-26 13:32 ` J. Bruce Fields
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