Linux NFS
Internal
Concepts
Locking Considerations
NFSv3 does not time out the lock held by a process that crashes. NFSv4 locks time out in the same situation.
Root Squash
Root squash represents reduction of the access rights for the remote superuser (root) when using identity authentication (local user is the same as remote user). If using root squash, the remote user is no longer super user on the NFS server. After implementing the root squash, the authorized superuser performs restricted actions after logging into an NFS server directly and not just by mounting the exported NFS folder.
NFS Caching
The nfs client mount option “ac” causes the client to cache file attributes. “ac” is the default as well. Changing it to not cache attributes “noac” incurs a significant performance penalty. The max time, by default that the nfs client caches file or directory attributes is 30 sec min, and 60 sec maximum, by default.
Asynchronous NFS
The default export behavior for both NFS Version 2 and Version 3 protocols, used by exportfs in nfs-utils versions prior to nfs-utils-1.0.1 is "asynchronous". This default permits the server to reply to client requests as soon as it has processed the request and handed it off to the local file system, without waiting for the data to be written to stable storage. This is indicated by the async option denoted in the server's export list. It yields better performance at the cost of possible data corruption if the server reboots while still holding unwritten data and/or metadata in its caches. This possible data corruption is not detectable at the time of occurrence, since the async option instructs the server to lie to the client, telling the client that all data has indeed been written to the stable storage, regardless of the protocol used.
More details: http://nfs.sourceforge.net/nfs-howto/ar01s05.html#sync_versus_async