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Setting up a high-availability pair for an NFS fileserver is only half of the battle. You ll need to be sure that your NFS clients are tolerant of breaks in service and continue to retry requests and recover gracefully from server outages. Again, the clients don t really see the difference between a single server reboot and a failover; your goal is to use failover to make the time to client recovery as short as possible. You also want to be sure there are clients at the ready.
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The default mode for mounting NFS filesystems is with the hard option, indicating that the client should continue to retry operations until it receives an acknowledgment from the server. The alternative to hard mounts is soft ones, in which a request is retransmitted a few times and then the client gives up. Five retries is the default, although this parameter can be set as a mount option. NFS soft mounts and reliability do not mix. Data integrity has to have the highest precedence. NFS soft mounts do not ensure data integrity on the server or application correctness on the NFS client. If your goal is to make sure that servers recover quickly and clients continue from where they were waiting, you cannot use soft-mount operations to give the clients a chance to abort operations in progress. Some system administrators prefer soft mounts to hard ones because they allow clients to break operations that are stuck on NFS servers that have crashed. Note that this is precisely the behavior you re seeking to fix by investing in availability techniques in the first place. You want clients to be as insensitive to the server s state, and even the server s identity, whenever possible. Mixing hard mounts and the intr mount option to allow NFS operations to be interrupted may give you control of the NFS clients more quickly than a server failover, but it kills the application process that was hung on the nonresponding server. Again, your goal should be to improve server reliability to the point where client-side behavior isn t your concern. You can focus your attention on a room of NFS servers, but it s nearly impossible to scale up your system administration staff to handle floors of NFS clients in various states of hanging and abnormally terminated applications. What happens if you use soft mounts, and a client request times out The process making the system call to the filesystem that was converted into an NFS call will eventually run into trouble. On a call to get the inode information from a file, possibly as the result of an ls command or part of the file opening sequence, a null structure will be returned to the calling process. A well-written
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process will detect an empty structure and complain (see Boundary Condition Checks in 13, Application Design ). Most applications, however, will dump core when they attempt to use the zero-filled values in that file information structure. Similarly, an attempt to read a page from a file that fails returns a zero-filled page; a failed write operation leaves a hole in the file on the server. Both of these failed operations cause data corruption. Writes that fail on soft mounts are particularly tricky to identify because the error may be returned to the calling process long after the error occurs (again, see the following text for more details on why this happens). Big rule with NFS servers: If you make them highly available, make the clients highly patient and persistent in sending requests until they are serviced. Soft mounts are a poor man s way of regaining client control; they were introduced in the early days of NFS when server uptime was measured in days, not months. With improvements in operating system reliability, network engineering and hardware improvements such as switches, and the introduction of server-side redundancy, soft mounts no longer really have a place in NFS environments. You ll get better predictability from server-side controls, and you ll only get control over data integrity with hard mounts.
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