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212 • Linux Symposium 2004 • Volume One plementation. In particular, while the protocol may use string names, the kernel still needs to deal with uid’s, so it must map between NFSv4 string names and integers. As with rpcsec_gss context initation, we solve this problem by making upcalls to a userspace daemon; with the mapping in userspace, it is easy to use mechanisms such as NIS or LDAP to do the actual mapping without introducing large amounts of code into the kernel. So as not to degrade performance by requiring a context switch every time we process a packet carrying a name, we cache the results of this mapping in the kernel. 3.6 Delegations NFSv4, like previous versions of NFS, does not attempt to provide full cache consistency. Instead, all that is guaranteed is that if an open follows a close of the same file, then data read after the open will reflect any modifications performed before the close. This makes both open and close potentially high latency operations, since they must wait for at least one round trip before returning–in the close case, to flush out any pending writes, and in the open case, to check the attributes of the file in question to determine whether the local cache should be invalidated. Locks provide similar semantics—writes are flushed on unlock, and cache consistency is verified on lock—and hence lock operations are also prone to high latencies. To mitigate these concerns, and to encourage the use of NFS’s locking features, delegations have been added to NFSv4. Delegations are granted or denied by the server in response to open calls, and give the client the right to perform later locks and opens locally, without the need to contact the server. A set of callbacks is provided so that the server can notify the client when another client requests an open that would confict with the open originally obtained by the client. Thus locks and opens may be performed quickly by the client in the common case when files are not being shared, but callbacks ensure that correct close-to-open (and unlock-to-lock) semantics may be enforced when there is contention. To allow other clients to proceed when a client holding a delegation reboots, clients are required to periodically send a “renew” operation to the server, indicating that it is still alive; a client that fails to send a renew operation within a given lease time (established when the client first contacts the server) may have all of its delegations and other locking state revoked. Most implementations of NFSv4 delegations, including Linux’s, are still young, and we haven’t yet gathered good data on the performance impact. Nevertheless, further extensions, including delegations over directories, are under consideration for future versions of the protocol. 3.7 ACLs ACL support is integrated into the protocol, with ACLs that are more similar to those found in NT than to the POSIX ACLs supported by Linux. Thus while it is possible to translate an arbitrary Linux ACL to an NFS4 ACL with nearly identical meaning, most NFS ACLs have no reasonable representation as Linux ACLs. Marius Eriksen has written a draft describing the POSIX to NFS4 ACL translation. Currently the Linux implementation uses this mapping, and rejects any NFS4 ACL that isn’t exactly in the image of this mapping. This en-
Linux Symposium 2004 • Volume One • 213 sures userland support from all tools that currently support POSIX ACLs, and simplifies ACL management when an exported filesystem is also used by local users, since both nfsd and the local users can use the backend filesystem’s POSIX ACL implementation. However it makes it difficult to interoperate with NFSv4 implementations that support the full ACL protocol. For that reason we will eventually also want to add support for NFSv4 ACLs. 4 Acknowledgements and Further Information This work has been sponsored by Sun Microsystems, Network Appliance, and the Accelerated Strategic Computing Initiative (ASCI). For further information, see www. citi.umich.edu/projects/nfsv4/.
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Proceedings of the Linux Symposium
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The State of ACPI in the Linux Kern
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Conference Organizers Andrew J. Hut
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TCP Connection Passing Werner Almes
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Linux Symposium 2004 • Volume One
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Cooperative Linux Dan Aloni da-x@co
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Build your own Wireless Access Poin
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Linux Block IO—present and future
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Linux AIO Performance and Robustnes
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Methods to Improve Bootup Time in L
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Linux on NUMA Systems Martin J. Bli
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Improving Kernel Performance by Unm
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Linux Virtualization on IBM POWER5
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The State of ACPI in the Linux Kern
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Scaling Linux® to the Extreme From
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Get More Device Drivers out of the
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Demands, Solutions, and Improvement
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Hotplug Memory and the Linux VM Dav
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