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). > > 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. > > 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 :-( NeilBrown - 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
