Re: [RFC PATCH v1 00/30] fs: inode->i_version rework and optimization

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Mar 30, 2017 at 07:11:48AM -0400, Jeff Layton wrote:
> On Thu, 2017-03-30 at 08:47 +0200, Jan Kara wrote:
> > Hum, so are we fine if i_version just changes (increases) for all inodes
> > after a server crash? If I understand its use right, it would mean
> > invalidation of all client's caches but that is not such a big deal given
> > how frequent server crashes should be, right?

Even if it's rare, it may be really painful when all your clients are
forced to throw out and repopulate their caches after a crash.  But,
yes, maybe we can live with it.

> > Because if above is acceptable we could make reported i_version to be a sum
> > of "superblock crash counter" and "inode i_version". We increment
> > "superblock crash counter" whenever we detect unclean filesystem shutdown.
> > That way after a crash we are guaranteed each inode will report new
> > i_version (the sum would probably have to look like "superblock crash
> > counter" * 65536 + "inode i_version" so that we avoid reusing possible
> > i_version numbers we gave away but did not write to disk but still...).
> > Thoughts?

How hard is this for filesystems to support?  Do they need an on-disk
format change to keep track of the crash counter?  Maybe not, maybe the
high bits of the i_version counters are all they need.

> That does sound like a good idea. This is a 64 bit value, so we should
> be able to carve out some upper bits for a crash counter without risking
> wrapping.
> 
> The other constraint here is that we'd like any later version of the
> counter to be larger than any earlier value that was handed out. I think
> this idea would still satisfy that.

I guess we just want to have some back-of-the-envelope estimates of
maximum number of i_version increments possible between crashes and
maximum number of crashes possible over lifetime of a filesystem, to
decide how to split up the bits.

I wonder if we could get away with using the new crash counter only for
*new* values of the i_version?  After a crash, use the on disk i_version
as is, and put off using the new crash counter until the next time the
file's modified.

That would still eliminate the risk of accidental reuse of an old
i_version value.  It still leaves some cases where the client could fail
to notice an update indefinitely.  All these cases I think have to
assume that a writer made some changes that it failed to ever sync, so
as long as we care only about close-to-open semantics perhaps those
cases don't matter.

I wonder if repeated crashes can lead to any odd corner cases.

--b.