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 - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
