On Tue, Mar 21, 2017 at 01:23:24PM -0400, Jeff Layton wrote:
> On Tue, 2017-03-21 at 12:30 -0400, J. Bruce Fields wrote:
> > - It's durable; the above comparison still works if there were reboots
> > between the two i_version checks.
> > - I don't know how realistic this is--we may need to figure out
> > if there's a weaker guarantee that's still useful. Do
> > filesystems actually make ctime/mtime/i_version changes
> > atomically with the changes that caused them? What if a
> > change attribute is exposed to an NFS client but doesn't make
> > it to disk, and then that value is reused after reboot?
> >
>
> Yeah, there could be atomicity there. If we bump i_version, we'll mark
> the inode dirty and I think that will end up with the new i_version at
> least being journalled before __mark_inode_dirty returns.
So you think the filesystem can provide the atomicity? In more detail:
- if I write to a file, a simultaneous reader should see either
(old data, old i_version) or (new data, new i_version), not a
combination of the two.
- ditto for metadata modifications.
- the same should be true if there's a crash.
(If that's not possible, then I think we could live with a brief window
of (new data, old i_version) as long as it doesn't persist beyond sync?)
> That said, I suppose it is possible for us to bump the counter, hand
> that new counter value out to a NFS client and then the box crashes
> before it makes it to the journal.
>
> Not sure how big a problem that really is.
The other case I was wondering about may have been unclear. Represent
the state of a file by a (data, i_version) pair. Say:
- file is modified from (F, V) to (F', V+1).
- client reads and caches (F', V+1).
- server crashes before writeback, so disk still has (F, V).
- after restart, someone else modifies file to (F'', V+1).
- original client revalidates its cache, sees V+1, concludes
file data is still F'.
This may not cause a real problem for clients depending only on
traditional NFS close-to-open semantics.
--b.
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