On Sat, 2017-12-16 at 11:18 -0500, Jeffrey Altman wrote: > Hi Jeff, > > A few thoughts on AFS usage below which might impact a future revision > of the API. I hope they are useful. > > On 12/16/2017 8:49 AM, Jeff Layton wrote: > > On Sat, 2017-12-16 at 08:46 -0500, Jeff Layton wrote: > > > From: Jeff Layton <jlayton@xxxxxxxxxx> > > > > > > For AFS, it's generally treated as an opaque value, so we use the > > > *_raw variants of the API here. > > > > > > Note that AFS has quite a different definition for this counter. AFS > > > only increments it on changes to the data, not for the metadata. We'll > > > need to reconcile that somehow if we ever want to present this to > > > userspace via statx. > > > > > From the patch series notes: > > "The inode->i_version field is supposed to be a value that changes > whenever there is any data or metadata change to the inode. Some > filesystems use it internally to detect directory changes during > readdir. knfsd will use it if the filesystem has MS_I_VERSION set. IMA > will also use it to optimize away some remeasurement if it's available. > NFS and AFS just use it to store an opaque change attribute from the > server. > > "Only btrfs, ext4, and xfs increment it for data changes. Because of > this, these filesystems must log the inode to disk whenever the > i_version counter changes. That has a non-zero performance impact, > especially on write-heavy workloads, because we end up dirtying the > inode metadata on every write, not just when the times change. [1]" > > > The AFS/AuriStorFS data version is an unsigned 64-bit value that is > incremented by the file server as part of a data changing operation. For > files, a StoreData and for directories entry manipulations such as > create, rename, delete. This data version is used to tag the version of > any subset of the data stream for caching and replication purposes. > > As Jeff notes, the AFS data version is not incremented for metadata > changes. Metadata cannot be trusted by clients without acquiring a > callback promise from a fileserver. The callback promise will either be > satisfied by the issuing fileserver sending a CallBack notification that > the metadata is no longer valid OR the callback promise will expire. > > Something else that is important to note that it is assumed that local > data changes that occur under a valid callback promise is newer than the > data on the fileserver. It might be useful if the new i_version API > supported major and minor version numbers. AFS implementations would > store the fileserver provided data version number as the major version > and would increment the minor version when local changes have been made > which have yet to be stored back to the fileserver. This functionality > would be especially useful if disconnected operations were implemented > for the AFS implementation. > > It might also be useful to separate metadata version and data version > although some filesystems would set the same value to both. For AFS, > the metadata major version would the timestamp at which the callback was > issued. > > Jeffrey Altman Thanks. That seems like rather specialized use case. If we did want to go that route, we'd probably need to give filesystems a way to overload how i_version is handled and queried (maybe some new inode ops?). Given that nothing ever looks at the the i_version in kAFS now, I don't have a lot of incentive to do anything along those lines in this set. I think this patchset will probably make it simpler to do something like that in the future, if you were motivated to do so though. -- Jeff Layton <jlayton@xxxxxxxxxx>
