CITI Experience with Directory Delegations

From Linux NFS

NOTE: this is a rough work-in-progress and will be fleshed-out over the next few days; thank you.

Directory Delegations Background

NFSv4.1 introduces read-only directory delegations, a protocol addition intended to enable clients to perform more-aggressive caching. More specifically, the goal is essentially to allow clients to avoid excess GETATTR, ACCESS, and LOOKUP calls to the server by increasing the reliability with which clients are able to do directory entry caching (READDIR), name caching (LOOKUP), and directory metadata caching (ACCESS and GETATTR).

The following quoted subsections are from Section 11 of the NFSv4.1 minor version draft:

NFSv4 client caching behavior

"Directory caching for the NFS version 4 protocol is similar to previous versions. Clients typically cache directory information for a duration determined by the client. At the end of a predefined timeout, the client will query the server to see if the directory has been updated. By caching attributes, clients reduce the number of GETATTR calls made to the server to validate attributes. Furthermore, frequently accessed files and directories, such as the current working directory, have their attributes cached on the client so that some NFS operations can be performed without having to make an RPC call. By caching name and inode information about most recently looked up entries in DNLC (Directory Name Lookup Cache), clients do not need to send LOOKUP calls to the server every time these files are accessed."

NFSv4.1 delegations extensions

"[The NFSv4] caching approach works reasonably well at reducing network traffic in many environments. However, it does not address environments where there are numerous queries for files that do not exist. In these cases of "misses", the client must make RPC calls to the server in order to provide reasonable application semantics and promptly detect the creation of new directory entries. Examples of high miss activity are compilation in software development environments. The current behavior of NFS limits its potential scalability and wide-area sharing effectiveness in these types of environments."

Furthermore, analysis of NFSv3 (whose client cache semantics NFSv4 mirrors) network traces by Brian Wickman at the University of Michigan (FIXME: need link to a copy of his prelim) show that a very surprising amount of NFS traffic are the periodic GETATTRs the clients send when a timeout triggers a cache revalidation.

At CITI, we are in the process of implementing directory delegations as described in Section 11 of the minor version draft, although we are not at this time implementing the notifications extension also described therein. The following are some specific aspects of the work.

Delegations and the Linux VFS Lease Subsystem

Directory delegations are implemented on the server with extensions to the Linux VFS file lease subsystem. A lease is a type of lock that gives the lease-holder the chance to perform any necessary tasks (e.g., flushing data) when an operation that conflicts with the lease-type is about to occur -- the caller who is causing the lease to break will block until the lease-holder signals that it is finished cleaning-up (or the lease is forcefully broken after a certain timeout).

The existing lease subsystem only works on files, and leases are only broken when a file is opened for writing or is truncated. In order to implement directory delegations, we have added support for directory leases. These will break when a leased directory is mutated by any additions, deletions, renames, or when the directory's own metadata changes (e.g., chown(1)). Note that changes to existing files, e.g., will not break directory leases.

However, for the very near-term, only NFS protocol operations break delegations. There are a couple operations that involve some tricky locking issues in the VFS which will be addressed. The difficulty is that, given that breaking a lease involves blocking the caller, one must ensure that no important locks -- like a directory inode's i_sem -- are held while the calling kernel thread blocks.

Leases are usually acquired via the fcntl(2) call, and a lease-holder usually receieve a signal from the kernel when a lease is being broken; the lease-holder indicates that any cleanup is finished with another fcntl(2) call. NFS leases are all acquired and revoked in-kernel.