On Thu, 2017-03-30 at 12:12 -0400, J. Bruce Fields wrote: > 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. > Yeah, assuming that normal reboots wouldn't cause this, then I don't see it as being too bad. > > > 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. > Yeah, I imagine we'd need a on-disk change for this unless there's something already present that we could use in place of a crash counter. > > 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 sounds difficult to get right. Suppose I have an inode that has not been updated in a long time. Someone writes to it and then queries the i_version. How do I know whether there were crashes since the last time I updated it? Or am I misunderstanding what you're proposing here? > 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. > -- Jeff Layton <jlayton@xxxxxxxxxx>
