(Lack of) i_version handling in udf

From: Steve Magnani <steve.magnani@digidescorp.com>
To: Jan Kara <jack@suse.cz>
Cc: Jeff Layton <jlayton@kernel.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: (Lack of) i_version handling in udf
Date: Fri, 22 Dec 2017 08:43:40 -0600	[thread overview]
Message-ID: <19f848dd-aa94-70bc-73b2-4c1d8b352636@digidescorp.com> (raw)
In-Reply-To: <20171222120556.7435-1-jlayton@kernel.org>

Jan,

Re: [PATCH v4 00/19] fs: rework and optimize i_version handling in filesystems

On 12/22/2017 06:05 AM, Jeff Layton wrote:

> The inode->i_version field is supposed to be a value that changes
> whenever there is any data or metadata change to the inode. Some
> filesystems use it internally to detect directory changes during
> readdir. knfsd will use it if the filesystem has MS_I_VERSION set. IMA
> will also use it to optimize away some remeasurement if it's available.
> NFS and AFS just use it to store an opaque change attribute from the
> server.
>
> Only btrfs, ext4, and xfs increment it for data changes. Because of
> this, these filesystems must log the inode to disk whenever the
> i_version counter changes. That has a non-zero performance impact,
> especially on write-heavy workloads, because we end up dirtying the
> inode metadata on every write, not just when the times change.
>
> It turns out though that none of these users of i_version require that
> it change on every change to the file. The only real requirement is that
> it be different if something changed since the last time we queried for
> it.
>
> If we keep track of when something queries the value, we can avoid
> bumping the counter and an on-disk update when nothing else has changed
> if no one has queried it since it was last incremented.

This happened to cross my radar, which made me think...is it a problem
that the UDF driver does not have any references to i_version at all?
I suppose R/W UDF is a small subset of the normal use cases, but the driver
does try to support it.

Regards,
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  Steven J. Magnani               "I claim this network for MARS!
  www.digidescorp.com              Earthling, return my space modulator!"

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