On Wed, 2024-02-07 at 15:02 +0000, Trond Myklebust wrote: > On Wed, 2024-02-07 at 09:56 -0500, Jeff Layton wrote: > > On Wed, 2024-02-07 at 14:21 +0000, Trond Myklebust wrote: > > > On Wed, 2024-02-07 at 08:34 -0500, Jeff Layton wrote: > > > > I've started work on a patchset to add support for directory > > > > delegations > > > > to the Linux kernel client and server. It's still too rough to > > > > post > > > > at > > > > this point, and for now, I'm just cobbling in a ioctl to drive > > > > it. > > > > > > > > As I started working on some of the client bits, however, I > > > > realized > > > > that I don't really have a clear picture as to when the client > > > > should > > > > request a delegation on a directory. It seems like there are a > > > > lot of > > > > things we could do: > > > > > > > > One idea: request one on an initial directory readdir. So maybe > > > > when > > > > the > > > > offset is 0 and we don't have a dir delegation already, do: > > > > > > > > PUTFH:GET_DIR_DELEGATION:READDIR > > > > > > > > Or, maybe just do it on any readdir when we haven't requested one > > > > in > > > > a > > > > little while? > > > > > > > > We could also do one on every lookup, when we expect that the > > > > result > > > > will be a directory. I'm not sure if LOOKUP_DIRECTORY would be a > > > > sufficient indicator or if we'd need the vfs to indicate that > > > > with a > > > > new > > > > flag. > > > > > > > > Would we also want to request one after a mkdir? > > > > > > > > PUTFH:CREATE:GET_DIR_DELEGATION:GETFH:GET_DIR_DELEGATION > > > > :... > > > > > > > > Assuming we can get this all working, what should drive the > > > > client to > > > > issues GET_DIR_DELEGATION ops? > > > > > > As far as I'm concerned, the main case to be made for directory > > > delegations in the client is for reducing the number of > > > revalidations > > > on said directory, particularly during path lookups. > > > i.e. the goal is to eliminate the need to constantly poll the > > > directory > > > change attribute, and to eliminate the need to constantly > > > revalidate > > > the dentries (and negative dentries!) contained in the directory > > > after > > > a change. > > > > > > Perhaps that means we should focus on adding a request for a > > > directory > > > delegation to the function nfs_lookup_revalidate() since that would > > > seem to indicate that we're going through the same directory > > > multiple > > > times? The other call site to consider would be > > > nfs_check_verifier(). > > > > > > > Sounds good. I'm not nearly at the point where I'm modifying client > > behavior yet, but I'll plan to try wiring it up in the revalidate > > codepaths first. > > Understood, but you appeared to be asking which COMPOUNDs to modify. I > think a discussion around the goals of introducing directory > delegations needs to inform that choice. > The goal is to improve lookup performance, and reduce the GETATTR load on directories. What sort of userland behavior should trigger the client to get a dir_deleg? Trying to improve repeated lookups in the same dir does seem like the biggest initial win for this. Plumbing one into readdir might also be reasonable. Someone doing a readdir is probably interested in the contents. If we get a deleg first, then that might allow the client to mark the directory "complete" once it has read every entry. -- Jeff Layton <jlayton@xxxxxxxxxx>
