linux-ext4 - Re: [patch 2/2] i_version update

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Message-ID: <20070529225817.GE5181@schatzie.adilger.int>
Date:	Tue, 29 May 2007 16:58:17 -0600
From:	Andreas Dilger <adilger@...sterfs.com>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	jean-noel.cordenner@...l.net, linux-ext4@...r.kernel.org,
	nfsv4@...ux-nfs.org, linux-fsdevel@...r.kernel.org
Subject: Re: [patch 2/2] i_version update - ext4 part

On May 29, 2007  12:44 -0700, Mingming Cao wrote:
> I am a little bit confused about the two patches. 
> 
> It appears in the ext4_expand_inode_extra_isize patch by Kalpak, there a
> new 64 bit i_fs_version field is added to ext4 inode structure for inode
> versioning support. read/store of this counter are properly handled, but
> missing the inode versioning counter update.

For the Lustre use of the inode version we don't care about the VFS changes
to i_version.  In fact - we want to be able to control the changes to
inode version ourselves so that e.g. file defragmenting or atime updates
don't change the inode version, and that recovery can restore the version
to a known state along with the rest of the metadata.

That said, since Lustre isn't in the kernel and we patch our version of
ext3 anyways it doesn't really matter what is done for NFS.  We will just
patch in our own behaviour if the final ext4 code isn't suitable in all
of the details.  Having 99% of the code the same at least makes this a
lot less work.

> But later in the second patch by Jean Noel, he re-used the VFS inode-
> >i_version for ext4 inode versioning, the counter is being updated every
> time the file is being changed. 

I don't know what the NFS requirements for the version are.  There may
also be some complaints from others if the i_version is 64 bits because
this contributes to generic inode growth and isn't used for other
filesystems.

> To me, i_fs_version and inode_version are the same thing, right?
> Shouldn't we choose one(I assume inode i_version?), and combine these
> two patch together? How about split the inode versioning part from the
> ext4_expand_inode_extra_isize patch(it does multiple things, and
> i_versioning doesn't longs there) and put it together with the rest of
> i_version update patches?

I don't have an objection to that, but I don't think it is required.

> BTW, how could NFS/user space to access the inode version counter?

If the Bull patch uses i_version then knfsd can just access it directly.
I don't think there is any API to access it from userspace.  One option
is to add a virtual EA like user.inode_version and have the kernel fill
this in from i_version.

Lustre will manipulate the ei->i_fs_version directly.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.

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