Re: [EXT4 set 4][PATCH 1/5] i_version:64 bit inode version

[Trond Myklebust]

On Wed, 2007-07-11 at 13:21 +1000, Neil Brown wrote:
> On Tuesday July 10, akpm@xxxxxxxxxxxxxxxxxxxx wrote:
> > 
> > Yes, thanks.  It doesn't actually tell us why we want to implement
> > this attribute and it doesn't tell us what the implications of failing
> > to do so are, but I guess we can take that on trust from the NFS guys.
> 
> You would like to think so, but remember NFSv4 was designed by a
> committee :-)
> 
> The 'change' number is used for cache consistency, and as the spec
> makes very strong statements about the 'change' number, it is very
> hard (or impossible) to implement a server correctly without storing a
> change number in stable storage (just one of my grips about V4).

Well... It reflects a requirement that was just as present in the
caching models that we use for NFSv2/v3, but that we glossed over for
other reasons.
The real difference here is that v2/v3 caching model assumes that you
have sufficient resolution in the ctime/mtime to allow clients to detect
any changes to the file/directory contents, whereas NFSv4 allows you to
use an arbitrary variable (that may be the ctime, if it has sufficient
resolution) for the same purposes.

> > But I suspect the ext4 implementation doesn't actually do this.  afaict we
> > won't update i_version for file overwrites (especially if s_time_gran can
> > indeed be 1,000,000,000) and of course for MAP_SHARED modifications.  What
> > would be the implications of this?
> 
> The first part sounds like a bug - i_version should really be updated
> by every call to ->commit_write (if that is still what it is called).
> 
> The MAP_SHARED thing is less obvious.  I guess every time we notice
> that the page might have been changed, we need to increment i_version.

You need to increment is any time that you detect remotely visible
changes.
IOW: any change that posix mandates should result in a ctime update,
must also result in an update of i_version. The only difference is that
i_version must not suffer from the time resolution issues that ctime
does.

> > And how does the NFS server know that the filesystem implements i_version? 
> > Will a zero-value of i_version have special significance, telling the
> > server to not send this attribute, perhaps?
> 
> That is a very important question.  Zero probably makes sense, but
> what ever it is needs to be agreed and documented.
> And just by-the-way, the server doesn't really have the option of not
> sending the attribute.  If i_version isn't defined, it has to fake
> something using mtime, and hope that is good enough.
> 
> Alternately we could mandate that i_version is always kept up-to-date
> and if a filesystem doesn't have anything to load from storage, it
> just sets it to the current time in nanoseconds.
> 
> That would mean that a client would need to flush it's cache whenever
> the inode fell out of cache on the server, but I don't think we can
> reliably do better than that.
> 
> I think I like that approach.
> 
> So my vote is to increment i_version in common code every time any
> change is made to the file, and alloc_inode should initialise it to
> current time, which might be changed by the filesystem before it calls
> unlock_new_inode. 
> ... but doesn't lustre want to control its i_version... so maybe not :-(

If lustre wants to be exportable via pNFS (as Peter Braam has suggested
it should), then it had better be able to return a change attribute that
is compatible with the NFSv4.1 spec...

Trond

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