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

[Jeff Layton]

On Fri, 2022-09-16 at 02:54 -0400, Theodore Ts'o wrote:
> On Fri, Sep 16, 2022 at 08:23:55AM +1000, NeilBrown wrote:
> > > > If the answer is that 'all values change', then why store the crash
> > > > counter in the inode at all? Why not just add it as an offset when
> > > > you're generating the user-visible change attribute?
> > > > 
> > > > i.e. statx.change_attr = inode->i_version + (crash counter * offset)
> 
> I had suggested just hashing the crash counter with the file system's
> on-disk i_version number, which is essentially what you are suggested.
> 
> > > Yes, if we plan to ensure that all the change attrs change after a
> > > crash, we can do that.
> > > 
> > > So what would make sense for an offset? Maybe 2**12? One would hope that
> > > there wouldn't be more than 4k increments before one of them made it to
> > > disk. OTOH, maybe that can happen with teeny-tiny writes.
> > 
> > Leave it up the to filesystem to decide.  The VFS and/or NFSD should
> > have not have part in calculating the i_version.  It should be entirely
> > in the filesystem - though support code could be provided if common
> > patterns exist across filesystems.
> 
> Oh, *heck* no.  This parameter is for the NFS implementation to
> decide, because it's NFS's caching algorithms which are at stake here.
> 
> As a the file system maintainer, I had offered to make an on-disk
> "crash counter" which would get updated when the journal had gotten
> replayed, in addition to the on-disk i_version number.  This will be
> available for the Linux implementation of NFSD to use, but that's up
> to *you* to decide how you want to use them.
> 
> I was perfectly happy with hashing the crash counter and the i_version
> because I had assumed that not *that* much stuff was going to be
> cached, and so invalidating all of the caches in the unusual case
> where there was a crash was acceptable.  After all it's a !@#?!@
> cache.  Caches sometimmes get invalidated.  "That is the order of
> things." (as Ramata'Klan once said in "Rocks and Shoals")
> 
> But if people expect that multiple TB's of data is going to be stored;
> that cache invalidation is unacceptable; and that a itsy-weeny chance
> of false negative failures which might cause data corruption might be
> acceptable tradeoff, hey, that's for the system which is providing
> caching semantics to determine.
> 
> PLEASE don't put this tradeoff on the file system authors; I would
> much prefer to leave this tradeoff in the hands of the system which is
> trying to do the caching.
> 

Yeah, if we were designing this from scratch, I might agree with leaving
more up to the filesystem, but the existing users all have pretty much
the same needs. I'm going to plan to try to keep most of this in the
common infrastructure defined in iversion.h.

Ted, for the ext4 crash counter, what wordsize were you thinking? I
doubt we'll be able to use much more than 32 bits so a larger integer is
probably not worthwhile. There are several holes in struct super_block
(at least on x86_64), so adding this field to the generic structure
needn't grow it.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>