Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field

[Dave Chinner]

On Tue, Sep 13, 2022 at 11:49:03AM +1000, NeilBrown wrote:
> On Tue, 13 Sep 2022, Dave Chinner wrote:
> > On Mon, Sep 12, 2022 at 07:42:16AM -0400, Jeff Layton wrote:
> > > On Sat, 2022-09-10 at 10:56 -0400, J. Bruce Fields wrote:
> > > > On Fri, Sep 09, 2022 at 12:36:29PM -0400, Jeff Layton wrote:
> > > > Our goal is to ensure that after a crash, any *new* i_versions that we
> > > > give out or write to disk are larger than any that have previously been
> > > > given out.  We can do that by ensuring that they're equal to at least
> > > > that old maximum.
> > > > 
> > > > So think of the 64-bit value we're storing in the superblock as a
> > > > ceiling on i_version values across all the filesystem's inodes.  Call it
> > > > s_version_max or something.  We also need to know what the maximum was
> > > > before the most recent crash.  Call that s_version_max_old.
> > > > 
> > > > Then we could get correct behavior if we generated i_versions with
> > > > something like:
> > > > 
> > > > 	i_version++;
> > > > 	if (i_version < s_version_max_old)
> > > > 		i_version = s_version_max_old;
> > > > 	if (i_version > s_version_max)
> > > > 		s_version_max = i_version + 1;
> > > > 
> > > > But that last step makes this ludicrously expensive, because for this to
> > > > be safe across crashes we need to update that value on disk as well, and
> > > > we need to do that frequently.
> > > > 
> > > > Fortunately, s_version_max doesn't have to be a tight bound at all.  We
> > > > can easily just initialize it to, say, 2^40, and only bump it by 2^40 at
> > > > a time.  And recognize when we're running up against it way ahead of
> > > > time, so we only need to say "here's an updated value, could you please
> > > > make sure it gets to disk sometime in the next twenty minutes"?
> > > > (Numbers made up.)
> > > > 
> > > > Sorry, that was way too many words.  But I think something like that
> > > > could work, and make it very difficult to hit any hard limits, and
> > > > actually not be too complicated??  Unless I missed something.
> > > > 
> > > 
> > > That's not too many words -- I appreciate a good "for dummies"
> > > explanation!
> > > 
> > > A scheme like that could work. It might be hard to do it without a
> > > spinlock or something, but maybe that's ok. Thinking more about how we'd
> > > implement this in the underlying filesystems:
> > > 
> > > To do this we'd need 2 64-bit fields in the on-disk and in-memory 
> > > superblocks for ext4, xfs and btrfs. On the first mount after a crash,
> > > the filesystem would need to bump s_version_max by the significant
> > > increment (2^40 bits or whatever). On a "clean" mount, it wouldn't need
> > > to do that.
> > 
> > Why only increment on crash? If the filesystem has been unmounted,
> > then any cached data is -stale- and must be discarded. e.g. unmount,
> > run fsck which cleans up corrupt files but does not modify
> > i_version, then mount. Remote caches are now invalid, but i_version
> > may not have changed, so we still need the clean unmount-mount cycle
> > to invalidate caches.
> 
> I disagree.  We do need fsck to cause caches to be invalidated IF IT
> FOUND SOMETHING TO REPAIR, but not if the filesystem was truely clean.

<sigh>

Neil, why the fuck are you shouting at me for making the obvious
observation that data in cleanly unmount filesystems can be modified
when they are off line?

Indeed, we know there are many systems out there that mount a
filesystem, preallocate and map the blocks that are allocated to a
large file, unmount the filesysetm, mmap the ranges of the block
device and pass them to RDMA hardware, then have sensor arrays rdma
data directly into the block device. Then when the measurement
application is done they walk the ondisk metadata to remove the
unwritten flags on the extents, mount the filesystem again and
export the file data to a HPC cluster for post-processing.....

So how does the filesystem know whether data the storage contains
for it's files has been modified while it is unmounted and so needs
to change the salt?

The short answer is that it can't, and so we cannot make assumptions
that a unmount/mount cycle has not changed the filesystem in any
way....

> > IOWs, what we want is a salted i_version value, with the filesystem
> > providing the unique per-mount salt that gets added to the
> > externally visible i_version values.
> 
> I agree this is a simple approach.  Possible the best.
> 
> > 
> > If that's the case, the salt doesn't need to be restricted to just
> > modifying the upper bits - as long as the salt increments
> > substantially and independently to the on-disk inode i_version then
> > we just don't care what bits of the superblock salt change from
> > mount to mount.
> > 
> > For XFS we already have a unique 64 bit salt we could use for every
> > mount - clean or unclean - and guarantee it is larger for every
> > mount. It also gets substantially bumped by fsck, too. It's called a
> > Log Sequence Number and we use them to track and strictly order
> > every modification we write into the log. This is exactly what is
> > needed for a i_version salt, and it's already guaranteed to be
> > persistent.
> 
> Invalidating the client cache on EVERY unmount/mount could impose
> unnecessary cost.  Imagine a client that caches a lot of data (several
> large files) from a server which is expected to fail-over from one
> cluster node to another from time to time.  Adding extra delays to a
> fail-over is not likely to be well received.

HA fail-over is something that happens rarely, and isn't something
we should be trying to optimise i_version for.  Indeed, HA failover
is usually a result of an active server crash/failure, in which case
server side filesystem recovery is required before the new node can
export the filesystem again. That's exactly the case you are talking
about needing to have the salt change to invalidate potentially
stale client side i_version values....

If the HA system needs to control the salt for co-ordinated, cache
coherent hand-over then -add an option for the HA server to control
the salt value itself-. HA orchestration has to handle so much state
hand-over between server nodes already that handling a salt value
for the mount is no big deal. This really is not something that
individual local filesystems need to care about, ever.

-Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx